Hydrological Assessment of Star Carr and the ...

Journal of Wetland Archaeology 11, 2011, 36–55

Hydrological Assessment of Star Carr and the Hertford Catchment, Yorkshire, UK

Tony Brown, Chris Bradley, Tim Grapes and Ian Boomer

Abstract The survival of one of Europe’s most important Mesolithic sites, Star Carr, has been threatened owing to recent changes in its hydrology and associated changes in groundwater geochemistry. Before this study it was unclear what was controlling these changes, but possible causes were changes in precipitation regime (notably the frequency of drought), changes in groundwater abstraction, recent agricultural drainage or a combination of these factors. This paper evaluates the hydrology of Star Carr and its environs within the River Hertford sub-catchment of the River Derwent. Available hydrological data (precipitation, evapotranspiration, river flow and groundwater levels) were collated and used to characterise the River Hertford catchment and provide a hydrological and hydrogeological context for the site. The data were augmented by the insertion of 12 dipwells into the site which were used for both site monitoring and model testing. The monitoring included water abstraction for isotopic analyses (δ2H v δ18O). Two modelling approaches were undertaken: firstly, calculation of the Palmer Hydrological Drought Index (PHDI); and secondly, development of a three layer, finite difference groundwater model of the site using Modflow. The results strongly suggest that hydrological changes at Star Carr have not been caused by changes in precipitation patterns, although the water flowing through the site is largely of meteoric (rainfall) origin. Groundwater abstraction has also not had any impact upon the site.

Authors Addresses: A. G. Brown Geography and Environment, Shackleton Building, University of Southampton, Highfield Campus, Southampton SO17 1BJ Email: [email protected] (Corresponding Author) C. Bradley and I. Boomer School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT Emails: C.Bradley@bham. ac.uk [email protected] T. Grapes Consulting Groundwater Scientist, July Cottage, Chequers Street, East Ruston, Norwich, NR12 9JT Email: [email protected]

However, results from the groundwater model suggest that the dominant factor has been the insertion of under-drainage, which has lowered the water-table by >0.5m into the archaeologically sensitive zone. The study reported here shows how high the sensitivity of a small wetland is to local land management practices, and, significantly, illustrates how a hydrological assessment of wetland archaeological sites can be made even without a history of on-site monitoring. Keywords: Mesolithic archaeology, archaeological site management, wetland hydrology, desiccation, in situ preservation, under-drainage, PHDI, Modflow

Introduction Star Carr is one of the most important Early Mesolithic wetland sites in north-west Europe (Milner et al. 2011). Parts of Star Carr have been excavated by Clark (1954), Mellars (Mellars and Dark 1998) and most recently by Conneller and Milner (2007–2008) but, in total, probably not more than 20% of the site has been excavated. However, in many ways finds from Star Carr have defined a model and image of early Holocene hunter-gatherers that provide the basis of our understanding of human culture and subsistence in this period. One of the principal reasons for this has been the remarkable preservation of this wetland site with abundant wood and bone (including the famous antler frontlets) as well as lithics. It also represents a rich palaeoenvironmental archive that has added considerably to the interpretation of the site and our understanding of Mesolithic human-environment interactions (Dark 2004). By analogy with other wetlands it is quite possible that the site contains (or contained) other organic relics including birch-bark artefacts, rope, human bone and even artefacts made from resin (P. Mellars pers. comm. 2010). Such potentially artefact-rich, Early Mesolithic sites are rare and so have high archaeological and heritage value. Star Carr has also been placed within the context of the wider landscape by work undertaken by the Vale of Pickering Trust (Lane and Schadla-Hall forthcoming) and Taylor (2011). Recent excavations by Milner and Conneller (2007 and 2008) have alerted the archaeological community to evidence that the site may be undergoing rapid environmental degradation with the loss of organic materials particularly wood, bone and antler. This concern led to the commissioning of a stratigraphic and sedimentological study by English Heritage (Boreham 2009; 2011) and to a meeting in October 2008 which discussed the future of the site. This meeting discussed various options for Star Carr, including a rescue-type excavation and site preservation. However, even if a large-scale rescue-type excavation programme were to be undertaken, both the landowner and English Heritage would still need to identify practical ways of minimizing the future loss of the archaeological resource. The concern was that the site was very close to irreversible change and that in common with many floodplain sites it is hydrologically on, or just over, a ‘tipping-point’ (Brown 2008).

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The hydrological context of Star Carr The site of Star Carr lies close to the eastern end of the Vale of Pickering, which is an east–west strike valley controlled by the underlying geology. To the south of the Vale of Pickering is the upland area of the Yorkshire Wolds, underlain by the Chalk Group of Cretaceous age (Figure 1). The chalk is a major regional aquifer that discharges via springs at the base of its scarp slope, which forms the southern flank of the Vale. Beneath the Vale itself are relatively soft, low permeability clays of Cretaceous and Jurassic age that are overlain by glacial deposits of Pleistocene age comprising boulder clay and sands and gravels. Where present, the sands and gravels form a significant local aquifer. To the north of the Vale is the Corallian Limestone of Jurassic age, which is a major aquifer formation. The Corallian aquifer is likely to have some degree of

Figure 1. Simplified geological map of the River Hertford catchment and part of the River Derwent catchment. Derived from various sources including BGS data held under licence.

Hydrological Assessment of Star Carr

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hydraulic connection with the glacial sands and gravels beneath the very northernmost section of the Vale, but the Corallian sequence is down-faulted beneath the northern half of the Vale and is not present beneath the southern half of the Vale. Some data and the results of modelling suggest that parts of the Derwent system lose water into the Corallian aquifer (Carey and Chadha 1998) and this may be pertinent to the Vale of Pickering. However, the hydrogeology of the Star Carr area itself is not well known and given the very limited information available, it was imperative that all available data were collated and used in this first hydrological assessment of the site.

Methodology The methodology consisted of three major activities; firstly, collation of hydrological information on surface-water (including abstractions) and groundwater; secondly, water sampling and determination of isotopic composition to provide an indication of the sources of water entering the site; and lastly groundwater modelling to assess controls on site groundwater levels and inform future management decisions. The collation of existing hydrological data on Star Carr and its wider catchment was undertaken to provide a hydrological baseline for the area. The study area initially focused on the River Hertford catchment and the local sub-catchment around the site, with the aim of identifying any hydrological trends that could have affected Star Carr and investigating the linkage between Star Carr and the catchment hydrological system. Data were collected from all relevant bodies and the types and sources of information used are listed in Table 1. The meteorological data were obtained for calculation of the Palmer Hydrological Drought Index as described below. The remaining datasets relate to surface water data (including water abstractions), geological and hydrogeological data. Data Meteorological data River discharge data from three local gauging stations and River Hertford stage data Digital basemap data at 1:25 000 scale and 1:10,000 scale Digital datasets of drift and bedrock geology at 1:50,000 scale

Source British Atmospheric Data Centre (BADC) Environment Agency (EA)

Ordnance Survey (OS) (Edina, Digimap) British Geological Survey (BGS) (OneGeology, Edina) Stratigraphic and hydrogeological data from 12 nearby wells BGS and boreholes Stratigraphic information and groundwater level data from Yorwaste Ltd. monitoring boreholes at the nearby Seamer Carr landfill site Local groundwater level data from observation boreholes EA within 10km of the site Local dipwell and ditch water level data from the Cayton Scarborough Borough Council and Flixton Carrs Wetland Project* Water abstraction licenses located within 3km of the site EA Monitoring of 12 dipwells inserted at Star Carr in 2009 Authors/EH * This is a partnership project involving Natural England, the EA, Scarborough Borough Council, the Royal Society for the Protection of Birds and North Yorkshire County Council.

Table 1. Sources of hydrological data used in the Star Carr hydrological assessment.

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River flows and groundwater levels The Star Carr archaeological site lies adjacent to the man-made channel of the River Hertford, a tributary of the River Derwent, whose confluence with the Hertford lies ~5km west of the site. There are no gauging stations on the Hertford itself and the closest gauging stations to the site are at West Ayton and Low Marishes on the River Derwent and at Snainton Ings on the Brompton Beck. In order to define the flow characteristics of the River Hertford catchment, discharge data from local gauging stations were used in conjunction with catchment area statistics. The catchment areas were defined from the equivalent Water Resource Management Unit (WRMU) areas given in the local Catchment Abstraction Management Strategy (CAMS) document for the River Derwent (Environment Agency 2006). Using the discharge record from three local gauging stations, a synthetic discharge series was created, representing the Derwent catchment between West Ayton and Low Marishes, including the River Hertford catchment, but excluding the small catchment of the Brompton Beck. The catchment area for this synthetic discharge record was then calculated based upon the CAMS WRMU areas for Low Marishes (minus the Brompton Beck catchment) and the River Hertford, giving a total area of 342.5km2. The River Hertford WRMU area represented 23.9% of this area and a simple areal weighting was then used to calculate the discharge of the River Hertford. Summary statistics on the gauging stations (g/s) and the WRMU areas are given in Table 2. In order to assess whether the recent changes in the hydrology of Star Carr could be controlled by large-scale hydrogeological changes, groundwater levels from a number of observation boreholes in the chalk aquifer to the south of the Vale of Pickering and from the Corallian aquifer to the north of the Vale were examined.

Isotope analysis Analysis of hydrogen and oxygen isotopes (δ2H (or δD) vs δ18O) has been previously used to identify the water sources during the annual hydrological cycle in a small Catchment area description

G/s No.

Derwent at West 27048 Ayton g/s Brompton Beck at 27073 Snainton Ings g/s Derwent at Low 27087 Marishes g/s West Ayton WRMU River Hertford WRMU Low Marishes WRMU (excluding Brompton Beck catchment)

Gauging Catchstation ment area grid ref. (km2) SE 990 853 127.0

Mean flow Q95 (cumecs) (cumecs)

Q10 (cumecs)

0.47

0.026

1.1

SE 936 794

12.9

0.26

0.034

0.528

SE 833 774

457.5

3.83

0.913

7.604

124.5 81.7 260.8

0.737*

0.219*

1.440*

* calculated by areal weighting for the period July 1989 to November 2009.

Table 2. Catchment area details and discharge statistics.


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wetland in Shropshire (Bradley et al. 2007). Water is composed of hydrogen and oxygen. Hydrogen is present as two stable isotopes 1H, 2H while Oxygen is present as three stable isotopes 16O, 17O, 18O. The stable isotope composition of a water sample is described as the ratio of the heaviest/heavier isotope compared to the lightest (relative to a standard): oxygen is reported as δ18O while hydrogen is reported as δ2H. The relative proportions of lighter and heavier isotopes reflect physical processes such as evaporation and condensation, and are not directly influenced by the hydrochemistry (i.e. pH, electrical conductivity). A total of 77 water samples were analysed from surface and subsurface sources including an opportunistic suite of 4 precipitation samples that were taken during a site visit on 21–22 October 2009 when a rain-bearing front crossed the area. Water samples were taken on the afternoon of the 21 Oct. and again in the early morning, mid-morning and early afternoon of the 22nd. Each sample thus represents an average of the previous 2–3 hours of rainfall. The significant variation in their isotopic composition illustrates the typical processes of ‘rain-out’ as weather systems evolve. The samples taken from the Star Carr site can be classified into four main associations: 1. 2. 3. 4.

field drains (in the subsurface draining the main Star Carr field); surface ditches and rivers (includes field boundary ditches and river courses); dipwells (this study – placed during the course of this assessment); dipwells (Cayton and Flixton Carrs Project – pre-existing dipwells in adjacent fields)

Samples were analysed at the University of Birmingham Stable Isotope facility (School of GEES) following the methodology outlined in Bradley et al. (2007). All results are expressed using the δ convention, relative to the standard (Vienna Standard Mean Ocean Water, V-SMOW). For each sample, both the δ18O and δ2H isotope values were determined, which enables the data to be plotted on a traditional hydrological isotope graph together with the Global Meteoric Water Line (GMWL). Globally, all precipitation falls on or around the GMWL, but regional conditions often give rise to slightly different lines (Local, or LMWL) while evaporation takes the isotopic composition of a sample away from its original position on the LMWL at a lower slope.

Modelling Whilst it is possible to investigate medium- and long-term climatic impacts by solely considering local precipitation and estimated evapotranspiration, more powerful indices are available that can estimate local soil water storage as a function of weekly (or monthly) precipitation, and temperature, and which take into account variable water storage through a soil profile. This enables the longer-term consequences of significant hydrological events to be taken into account, whether associated with unusually wet, or dry, conditions, and placed in the context of the longer-term climate. One commonly used index, the Palmer Hydrological Drought Index (PHDI, Palmer 1965), incorporates antecedent precipitation, moisture supply, and moisture demand (i.e.

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evapotranspiration) into a water balance (Heim 2002). The index partitions precipitation between evapotranspiration, soil moisture storage and runoff and can be calculated over weekly or monthly time intervals to produce output values that range from +6 to -6 for extremely wet and dry conditions respectively. The PHDI index was chosen as a measure of possibly hydrological change as it represents a compromise between data requirements, which could be satisfied by existing records, and a more sophisticated treatment of soil storage than simple drought indices. The stages in calculating the PHDI are complex given the need to determine, retrospectively, the time at which periods of drought may be deemed to start or finish (Alley 1984). However, in essence the index envisages the soil to consist of two layers: an upper, top soil, which has a field capacity of 2.5cm, and a lower bottom layer (root zone) which only receives water when the upper soil is saturated. When both soil layers are saturated, the excess precipitation is considered to be lost as runoff (RO). A number of intermediate variables considered ‘climatologically appropriate for existing conditions’ (CAFEC) are calculated by the Index to enable periods that are unusually wet or dry to be identified. The stages in calculating the PHDI following Heim (2002) are: 1. 2. 3. 4. 5.

Complete a monthly/weekly water balance using: precipitation, evapotranspiration, soil moisture loss, recharge and runoff; Summarise to derive coefficients that depend upon the climate at a specific site; Repeat using derived coefficients to determine moisture required for ‘normal’ weather during each month / week (CAFEC) for each parameter; Determine precipitation departure for each month/week and convert to index of moisture anomaly (Z index); Analyse to determine the beginning, ending and severity of the drought periods.

The PHDI was calculated at Star Carr using a program developed by the former US National Agricultural Decision Support (Wells 2003). The model requires weekly (or monthly) temperature, and precipitation, and long-term average weekly (or monthly) temperatures. Also required are the ‘available water holding capacity’ of the soil; and the latitude of the site which is used to estimate Potential Evapotranspiration using the Thornthwaite method (Thornthwaite 1948). The PHDI was determined for Star Carr using the input files described in Table 3.

Groundwater modelling The process of developing a groundwater model for a site consists firstly of conceptualising the hydrology of the site; and secondly, representing this in a model which is essentially a formalised and parameterised simulation of reality. The model is then run to produce output results, usually in the form of simulated groundwater levels for the model domain. These simulated levels can be compared to actual groundwater levels recorded in the field (if available) to assess model accuracy. Normally a process of calibration takes place, by adjusting model parameters within acceptable limits, to produce an optimal representation of reality. Ideally the optimised model should

Hydrological Assessment of Star Carr Parameter Weekly Precipitation

Weekly Temperature

Mean Temperature Series Other parameters

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Description Daily precipitation totals were obtained from Ganton Golf Club, 5km SW of Star Carr from 2 Jan 1961 to 30 Sept 2009. Occasional missing values were infilled using daily precipitation data from Irton Pumping Station (4km NW) to produce a continuous daily precipitation series over the period. Weekly precipitation totals were then determined and formatted for input to the PHDI model. A temperature series comparable to precipitation, could only be obtained using daily temperature data from a weather station at High Mowthorpe (18km SW of Star Carr; 175m asl) from 2 Jan 1961 to 30 September 2009. High Mowthorpe is ~150m higher than Star Carr and daily temperatures were adjusted by +1° (taking into account the standard lapse rate of 6.5°C /1000m). Additional daily temperature series using the actual temperature data from High Mowthorpe, and the data plus 2°C were calculated to examine the sensitivity of the results to changes in temperature. Weekly mean temperatures were calculated and formatted for input to the PHDI model. Weekly mean temperatures were determined over the period January 1961 to September 2009 inclusive and were formatted for input to the PHDI model. The latitutude of Star Carr 52.21°N, and an estimate of the total Available Water Holding Capacity (AWC) of the soil, both within the plough layer (2.5cm) and at depth, were specified in a ‘parameter’ file. AWC was initially estimated at 25cm, and additional model runs were completed to investigate the effects of changes in the AWHC by ±25%.

Table 3. Data types and sources used for the PHDI modelling.

then be used to simulate another set of field data (independent from the data used for calibration) to confirm the accuracy of the model – a process known as verification. Once the model has been verified, selected parameters can be varied systematically to assess changes in the local hydrology and to derive possible future scenarios with respect to future climate changes and/or modifications in local hydrological management. The conceptual hydrogeological model of the site was based upon both existing data and limited results derived from the field work undertaken for this study. Detailed stratigraphic work undertaken in the Star Carr settlement area (Boreham 2009) was supplemented by fieldwork for this study, which provided information on the shallow subsurface formations in the model domain. Borehole data and British Geological Survey geological mapping, supplemented by data provided by Yorwaste for boreholes at the Seamer Carr landfill site, gave information on the deeper stratigraphy and the wider area around the site. The most important aspect of the conceptual model was to investigate how sufficient water had been retained in the deposits (peat, clay and gravel) underlying the Star Carr settlement area, in order to preserve the organic archaeology, even though it was in close proximity to the deep, artificially incised, channel of the River Hertford. Given that the sands and gravels underlying the peat are highly permeable, and in direct hydraulic contact with the river, it was essential that the conceptual model envisaged

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water in the peat deposits to be acting as a minor, perched aquifer, not in hydraulic contact with the underlying gravels. It was also essential to restrict lateral drainage from the peat deposits towards the river, otherwise the water-table in the settlement area would be drawn down by the presence of the river channel. Initially the perched water-table in the peat deposits was represented in the model by a very low vertical permeability in the peat layer, but this proved insufficient to produce the necessary effect. Consequently, the conceptual model was refined to incorporate another layer (a low permeability peat or clay), between the shallow peat and the underlying gravel, with a sufficiently low permeability to produce the perched water-table in the shallow peat. It was recognised that the basal section of the peat sequence was different from the shallow peat, there being sandy organic peat at shallow depths with wood peat below (Boreham 2009). Our conceptualisation of the peat sequence assumed that the wood peat was in a compressed state and thus characterised by a significantly lower permeability than the upper sandy peat, allowing it to act as an impermeable barrier at the base of the peat sequence, thereby producing a perched water-table in the shallow peat and locally confining the underlying sand and gravel deposit. This also allows the model to be applicable to areas upstream of Star Carr and similar sites within the palaeolake Flixton system, such as Seamer Carr where there are basal calcareous marls and blue lacustrine clays (Cloutman and Smith 1988). To restrict lateral drainage from the shallow peat towards the river, it was assumed that construction of the artificial channel of the River Hertford had disturbed the integrity of the shallow peat deposits immediately adjacent to the channel, reducing their effective permeability as a result of compaction and through mixing with lower permeability sediments derived from the channel excavation. The compacted and coarse nature of the river levee material (silty sand and gravel) was demonstrated at Star Carr as it was not possible to install dipwells by hand augering at a number of locations adjacent to the channel. The finalised conceptual model comprised: 1. 2. 3.

an unconfined, permeable upper peat layer (except in a narrow zone immediately adjacent to the river channel, where it was of lower permeability); a thin, relatively impermeable lower peat or clay layer; a thick, locally confined, highly permeable, sand and gravel layer.

Underlying the sand and gravel is the bedrock strata of the Lower Cretaceous Speeton Clay Formation and the Upper Jurassic Kimmeridge Clay Formation, both of which are of very low permeability and form an effectively impermeable base to the local groundwater flow system. The modelling was undertaken using Groundwater Vistas v5.35, a proprietary groundwater flow model with pre- and post-processors to aid the creation of input files and the display of output results. This model is based upon the Modflow (MODular Three-Dimensional Finite Difference Ground-Water FLOW Model) code described by McDonald and Harbaugh (1988) and Harbaugh and McDonald (1996). Modflow is a block-centred, quasi-three dimensional, finite difference groundwater model developed by the United States Geological Survey. The details of the model set up and parameterisation for both the base-line and drained runs are given in Table 4. The

Hydrological Assessment of Star Carr Model characteristic Time domain

Value(s) Steady state

Grid cell size

5m × 5m

Grid dimensions Model domain Grid orientation

200 columns, 200 rows 1000m × 1000m Vertical axis of model rotated 14° anticlockwise from Ordnance Survey grid north Perimeter boundary conditions East and West = No Flow North = Constant Head (24mOD) South = Constant Head (26.5mOD) Internal boundary conditions Drain cells for the River Hertford, surface drains and subsurface land drains Drain / river cell elevations Representative of actual elevations Layer details Layer thickness Horizontal hydraulic conductivity (Kx, Ky) Vertical hydraulic conductivity (Kz)

Layer 1 Layer 2 Layer 3 Layer 1 Layer 2 Layer 3 Layer 1 Layer 2

Layer 1 – shallow, sandy peat Layer 2 – deeper, compressed wood peat Layer 3 – basal sands and gravels 1.5m 0.5m 15 to 17m 3 (0.001 adjacent to river) 0.00015 10 0.3 (0.001 adjacent to river) 0.00015

Layer 3 1

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Details Produced snapshot of average conditions for specified parameter configuration Minimum size that allowed effective representation of smallest features (land drains at 10m separation)) Allowed whole of Star Carr site and its environs to be simulated Placed River Hertford channel parallel to model rows Based upon estimated groundwater flow lines and local groundwater level data Most appropriate representation of hydrological functioning Based upon on-site surveying of hydrological features Based upon local borehole logs, site-specific stratigraphic work (Boreham, 2009) and field work for this study Informed by hydraulic testing undertaken in the new dipwells installed on site, and estimated from published literature (Hobbs, 1986; Freeze and Cherry, 1979). Refined by model calibration

Table 4. Model set up and parameterisation.

initial parameterisation and base-line runs used the stratigraphic data, dimensions and all other parameters but with no under-drainage.

Results River flows and groundwater levels The synthetic discharge series calculated for the River Hertford catchment is shown in Figure 2a and indicates that the flow regime in the River Hertford has been relatively stable over the 20-year period of record. The period from 1989 to 1998 was characterised by discharges that were generally below the long-term average conditions, reflecting significant droughts in 1989–1992 and in 1995. Since 1998 the discharges have generally been average or above average, which suggests that there has been no catchment-wide reduction in surface water flows in the last 10 years. In order to assess the possible impact of local water abstractions, information on licensed abstractions within 3km of

Figure 2. a: Calculated discharge record for the River Hertford; b: An example of chalk aquifer groundwater levels from Ganton Dale observation borehole (located 6km south of Star Carr); c: An example of Corallian aquifer groundwater levels from Irton Old Well (located 4km NNW of Star Carr); d: Groundwater levels from Grove Farm observation borehole (located 1.7km NE of Star Carr) in the Pleistocene sand and gravel aquifer of the Vale of Pickering Water levels observed in selected dipwells at Star Carr (a downstream longitudinal section adjacent to the river) δ2H v δ18O for all samples, detailed plot (omitting precipitation event). Samples taken between October and December, 2009. Comparison of the PHDI and groundwater levels at Grove Farm Base case scenario: simulated groundwater contours in Layer 1 for the settlement area (in ~mOD) and (b) 2010 scenario model Layer 1 simulated groundwater contours for the settlement area (in ~mOD).

46 Tony Brown, Chris Bradley, Tim Grapes and Ian Boomer


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the Star Carr site were obtained from the Environment Agency (EA). Three abstractions were identified that could potentially affect the site, but as their combined daily licensed volumes represent less than 20% of the calculated Q95 discharge (the discharge that is exceeded 95% of the time) in the river they are relatively small compared to the river flow. Given that the calculated long-term pattern of discharge in the River Hertford is stable or increasing, there appears to be no abstraction-related impact on the river flow regime. Groundwater level records from an observation borehole at Ganton Dale (Grid reference TA 016 746, Figure 2b), 6.5km south of Star Carr, were used to illustrate the pattern of variability in the chalk aquifer while groundwater levels in the Corallian aquifer were described using records from Irton Old Well (TA 003 840, Figure 2c), 4km north-north-west of Star Carr. The long term pattern of groundwater response in both the chalk and Corallian aquifers is best seen in the two-year moving average trend line, and both aquifers display similar patterns over the period of record, with stable, moderate levels in the 1980s, low but variable levels in the 1990s and generally high, stable, levels from 2000 onwards. These results are similar to the findings from the River Hertford discharge record and indicate that there has been no regional-scale reduction in groundwater levels in the last 10 years. The thick Pleistocene sand and gravel deposits beneath the Vale of Pickering form a significant local aquifer and groundwater level data for this aquifer were obtained from several local EA observation boreholes and three Yorwaste monitoring boreholes at the Seamer Carr landfill site. An example of the groundwater response from a deep borehole in this aquifer at Grove Farm (1.7km north-east of Star Carr) is shown in Figure 2d. Two supplementary data sets of shorter duration, a shallow borehole at Seamer Carr, and shallow dipwells installed in peat deposits noth-east of Star Carr for the Cayton and Flixton Carrs Wetland Project (Scarborough Borough Council), provide a good analogue for the hydrology of Star Carr, and illustrate a pattern of response for peat deposits adjacent to surface ditches. The records are given in Brown et al. (2010) and in more detailed hydrological modelling (Bradley et al. in prep.).

Isotope analysis The large-scale and detailed plots of the isotope compositions of water samples (Figure 4) illustrate that all data points lie on or around the meteoric water line, and there is no clear evidence of evaporation indicating that surface waters at Star Carr are relatively young with short residence times. It is also clear that the chalk groundwaters (the isotope composition of which approximates mean annual precipitation) are relatively distinct from most other samples, plotting at the lower end of the range (black circles). Groundwater does not therefore appear to be a major source for most of the samples analysed. The closest values are from some of the dipwells which probably intersect the local groundwater-table. The field drains are generally lighter than groundwater samples but fall on the meteoric line, which is strongly indicative of a precipitation, rather than groundwater, source for the Star Carr field drains. The precipitation samples collected during a 24 hr period in October further support direct rainfall as the most likely source for most of the non-groundwater samples (both surficial and subsurface).

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Figure 3. Water levels observed in selected dipwells at Star Carr (a downstream longitudinal section adjacent to the river).

Figure 4. δ2H v δ18O for all samples, detailed plot (omitting precipitation event). Samples taken between October and December, 2009.


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There are no clearly defined temporal trends in any of the isotope or water chemistry proxies recorded during the study period. Hydrological/hydrochemical conditions can thus be considered to have remained relatively constant throughout this interval.

Water budget modelling Two of the model series generated are given as a time series in Figure 5a, using a midtemperature series (High Mowthorpe +1°C) and a higher temperature series (High Mowthorpe +2°C) over the period for which local climate data are available (January 1961–September 2009). Unusually wet conditions are indicated by values of the index >0; and dry conditions by values -2 to indicate droughts of moderate severity or greater, figure 6.2 suggests approximately seven such droughts have occurred at Star Carr since January 1961: July 1964–August 1965; July 1972–August 1973; March 1974–September 1974; June 1976–September 1976 March 1989–April 1991 May 1991–October 1993 December 1996–May 1998. The results indicate that conditions have been relatively wet (or at least not unusually dry) over the past 10 years. However, since 1961 there has been a succession of droughts, of which those from March 1989 to April 1991 (26 months) and from May 1991 to October 1993 (30 months), were of particularly long duration. Both summer and winter droughts are likely to have implications for soil-water conditions at Star Carr. Water-tables can be expected to fall significantly during summer droughts due to continued evapotranspiration at a time of low rainfall. Conversely, less groundwater recharge is likely during periods of winter droughts which could affect water-tables the following summer. It is also possible to compare the PHDI with other available hydrological time-series (e.g. dipwell observations and groundwater level data). In Figure 5b, groundwater levels from Grove Farm borehole (1.7km north-east of Star Carr) are plotted at the same scale

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Figure 5. Comparison of the PHDI and groundwater levels at Grove Farm.

as the PHDI. While there are some differences in the correspondence of high and low PHDI values and groundwater levels, both series respond very clearly to the drought from May 1989, with respect to both the timing and magnitude of the event. However, groundwater levels appear to recover more quickly after the event (from 1993) than the PHDI, and do not rise significantly during the period of high rainfall (from 2000 onwards). The latter probably indicates a local control on peak groundwater levels (e.g. a spring or other point of groundwater discharge). Whilst it would be possible to quantify the relationship between the PHDI and groundwater levels, it would be more useful to use the PHDI to generate a surrogate series of water-table variation at Star Carr. At present this is not possible, given the lack of water-level data, but should be feasible with continued monitoring of the dipwells now installed at Star Carr.


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Groundwater modelling The results of the base case model simulation are presented in Figure 6a as a contour map of groundwater levels in Layer 1 of the model showing levels falling from ~25.5mOD at the southern edge of the field to ~24mOD along the river. The complicated pattern in the bottom left corner of the figure is the condensed pattern of contours caused by a local reduction in groundwater levels as a result of the distribution of surface drains in this area. The perched groundwater in the shallow peat beneath the settlement area is at ~24.6mOD, giving a depth to the water-table at this point of between 0.4 and 0.9m (based on the ground level of ~25 to ~25.5mOD). Under this scenario, the organic archaeology at between 0.8 and 1.2mbgl (below ground level) would remain mostly saturated. Having produced a satisfactory base case simulation, the groundwater model was then used to test the effect of modified drainage in the Star Carr field (as a result of the installation of subsurface land drains in ~ 2001), and produce a simulation of current groundwater levels in the area. Groundwater contours for Layer 1 of this current scenario model are shown in Figure 6b. Comparing the results with Figure 6a it is suggested that the subsurface land drains installed in the Star Carr field have had an impact on groundwater levels in the area. In the base case scenario the bulk of the Star Carr field had groundwater levels between ~24.5 and ~25mOD, whilst in the current scenario most of the field has levels between ~24 and ~24.5mOD. The western end of the field has a water-table 0.5m, meaning that the water-table now lies at 0.9–1.4m below ground level in the settlement area. Under this scenario, significant sections of the organic archaeology (lying at depths between 0.8 and 1.2mbgl) are largely situated above the water-table in unsaturated conditions and at risk of degradation. In order to test the accuracy of the current scenario model, the simulated groundwater levels were compared with actual levels recorded from the 12 new dipwells installed for this study. The residual (observed level – simulated level) was calculated for each dipwell based upon actual levels recorded on 29 January 2010 and the results are all within 0.5m of observed levels, with 8 of the 12 results being within 0.2m, including the dipwell closest to the settlement site. The dipwell data were not used in the calibration of the model and thus provide a semi-independent verification dataset. The accurate simulation of observed water levels gives confidence that the current scenario model provides a realistic depiction of the present hydrology of the Star Carr site and its environs.

Discussion and conclusions The data collated for the River Hertford catchment for the period 1989–2010 indicate that the flow regime has been relatively stable over the last 20 years with discharges generally being above average over the last 10 years. There has been no catchment-wide reduction in surface flows over the last 10 years. Long-term groundwater level records from the regional chalk and Corallian aquifers show a similar pattern of response with

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Figure 6. Base case scenario. a: simulated groundwater contours in Layer 1 for the settlement area (in ~mOD); b: 2010 scenario model Layer 1 simulated groundwater contours for the settlement area (in ~mOD). above-average groundwater levels occurring over the last 10 years. Isotopic analysis of 77 samples from the archaeological site, the river, surface drains and chalk springs indicates that the water from the site has short residence times and is of meteoric origin. Therefore little of it appears to have been sourced from the regional chalk aquifer. Application of the Palmer Hydrological Drought Index (PHDI) for water balance modelling shows a clear temporal clustering of wet and dry periods over the last 50 years with the period since AD 2000 being predominantly wet. This analysis provides no evidence for a climate-driven desiccation of the site. Groundwater levels from Grove


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Farm (1.7km north-east of Star Carr) show a similar trend to the PHDI suggesting that such hydrological fluctuations modelled by the PHDI would under base case conditions be reflected in the water-table history of the Star Carr site. With continued monitoring of the groundwater levels at Star Carr it will be possible to quantify the relationship between PHDI and groundwater levels and create a surrogate series for Star Carr. Local scale groundwater modelling using Modflow under steady-state conditions clearly shows the hydrological effects of the under-drainage on the site through reducing the water-table to ~24.1mOD, over 0.5m below the base case scenario. This modelling was tested by comparing the residuals (observed level-simulated level) against the observed groundwater levels, with good results. All the above analyses strongly suggest that there is little or no direct connection between the Holocene peat deposits at Star Carr and the regional chalk aquifer. The hydrological analyses at the catchment scale, the local groundwater levels and the PHDI all show that the hydrological environment has been stable or wetter over the last 10 years and provides no evidence to support a climatically driven desiccation of the site over the last 10 years. Under these conditions the over-riding hydrological control of the site over the last few years has been the insertion of effective under-drainage which have caused increased oxidation of the peat. This oxidation has been linked to changes in the chemistry of the site detrimental to preservation of bones, wood and antler (Boreham et al. 2011; Milner et al. 2011). The hydrological isolation of the site from the chalk aquifer coupled with the absence of low pH in groundwater suggest that the source of the hyper-acidity at the site is due to ‘authigenic’ acidification by pyrite oxidation, a process known to occur in floodplain wetlands (Brown et al. 2010) and confirmed at this site by Boreham (2009) and Boreham et al. (2011). It is suggested that the source of this sulphide is pyrite reworked from the Kimmeridge Clay into the Late Pleistocene and Holocene clays which underlie the peat in many places. Though there may also be a contribution from bacterial reduction of Fe within these water logged sediments in the presence of aerosol derived sulphate (event-driven, marine storm spray). This requires further investigation which should be undertaken as a geochemical component of continued site monitoring and hydrological modelling. Wetland archaeological sites have a potential value far above their intrinsic value owing to the preservation of perishable material and the significant contribution this can make to understanding past culture and particularly human subsistence and lifestyles. However, this value is easily threatened by both land management-induced and climatically-induced hydrological change to the small and often vulnerable wetlands in which they are preserved (Brown and Bradley 1995; Bradley and Brown 1996). This study shows how an assessment of the basic hydrological functioning of a wetland archaeological site can be made in only a few months using actual data, although it is also dependant on the routine monitoring of stream-flow and meteorology. This study also demonstrates the value of hydrological evaluation of a wetland site for understanding and assessing its preservation state and, more importantly, providing evidence to underpin future management decisions perhaps feeding into decisionmaking systems (Vorenhout 2011). This is essential if the principles enshrined under the International Valetta Treaty (Malta Convention) of 1996 promoting ‘the conservation and maintenance of the archaeological heritage, preferably in situ’ are to be fulfilled.

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Acknowedgements The authors must thank Laura Basell for her assistance with the production of the geological and topographic data, Tim Burkinshaw for assistance in the field and for the provision of hydrological data, Nicky Milner for advice and provision of data, Steve Boreham for provision of the stratigraphic and geochemical report, and Buzz Busby and Vanessa Straker for their help and support throughout the project. Additional thanks go to ESI Ltd. for provision of groundwater modelling software and the Coverdale family for access to their land and the site. This research was funded under the Historic Environment Enabling Fund by English Heritage (PNUM5822).

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