coastal sp numerical modelling

Monitoring seawater intrusion into a fractured coastal aquifer using measurements of self-potential (SP)

H31G-1511

Donald John MacAllister1,2, Matthew Jackson,1 Adrian Butler2, Jan Vinogradov3, Amadi Ijioma1 Department of Earth Science and Engineering, 2Department of Civil and Environmental Engineering, Imperial College London. 3School of Engineering, King’s College, University of Aberdeen.

1

INTRODUCTION

FIELD EXPERIMENT RESULTS – COASTAL SP

• Seawater intrusion is a global phenomenon occurring in coastal aquifers.

• The SP exhibits oscillations of c. 400mV amplitude (Fig. 4(a)) that are in phase and anti-correlated (Fig. 4(c)) with the head fluctuations in the borehole.

• In natural conditions, a freshwater wedge develops; less dense freshwater overlies more dense seawater. During abstraction, saline up-coning may cause contamination of the borehole (Fig.1). In highly fractured coastal aquifers, like the UK Chalk, saline water also can move laterally and rapidly towards abstraction boreholes. Real-time monitoring of saline water movement is essential for effective borehole management.

• The SP oscillations are observed even when no saline water (identified using conductivity measurements that are not shown) enters the borehole. We hypothesize that they are caused by (i) electrokinetic processes local to the borehole associated with the tidally driven head variations and/or (2) exclusionD DD diffusion processes associated with salinity (b) (a) gradients remote from the borehole, and tidally driven variations in the location of the saline front. Voltage (mV)

• The conditions created by seawater intrusion generate exclusion-diffusion potentials (𝑽𝑬𝑬𝑫 ) and electrokinetic potentials (𝑽𝑬𝑲 ), the sum of which will be observed as the overall self-potential (SP). Gradients in salinity cause 𝑽𝑬𝑬𝑫 and gradients in pressure cause 𝑽𝑬𝑲 . • Borehole SP measurements may allow remote monitoring of seawater intrusion into coastal aquifers and detect saline water movement towards an abstraction well. The aim of this study is to determine whether borehole SP measurements can be used to monitor saline intrusion into coastal aquifers. This poster reports the results of field experiments and numerical modelling focussed on the highly fractured UK Chalk aquifer. The objective of these is to understand the dynamics of SP in fractured coastal aquifers.

2013

Time (month)

2014

NUMERICAL MODELLING – COASTAL SP

• Two boreholes were instrumented in order to compare the difference in borehole SP between an inland site and a coastal site at risk of seawater intrusion. Coastal borehole Inland borehole Chalk

(a)

(b) Head against the smoothed borehole voltage. Head and voltage are anti-correlated and both have a period of 12.42hrs.

Figure 1. Salt water up-coning in a homogenous aquifer (USGS).

FIELD EXPERIMENTS - METHODS

(b)

Figure 4. (a) SP data referenced to the shallowest borehole electrode, smoothed using a second order Savitzky-Golay filter and a four hour window.

Permo-Triassic Sandstone Risk of saline intrusion

• A steady state saline wedge is developed. The seaward boundary is at seawater salinity and the head varies over a tidal cycle consistent with observed tidal data. The inland boundary is at groundwater salinity and has a constant head 1m above mean sea-level. • A fracture zone is modelled c. 80m below the top of the model that Hydrodynamic properties intersects the borehole, consistent with borehole geophysical data. Aquifer properties Tidal simulation boundaries 35.80

(b)

(d)

Salt 200m Conc. (g/l)

90.1

(c) Balsdean abstraction borehole

EK

0.38

Brighton

ED

3km Saltdean monitoring borehole

Bulk rock permeability (top 50m): 15D Exponential perm decrease to base: 2mD Fracture Permeability: 2500D Bulk rock porosity: 1% Fracture porosity: 2%

Pressure spring: -3.5mAOD to +3.5mAOD Pressure neap: -1.5mAOD to + 1.5mAOD Constant inland head: 0.5mAOD

Electrodynamic properties (from laboratory experiments) GW Coupling Coefficient: SW Coupling Coefficient: -0.575±0.080 mV/mH2O -0.0101±0.0020 mV/mH2O 19.30±7.3mV across the saline front.

NUMERICAL MODELLING RESULTS – COASTAL SP • The model SP versus time (Fig. 5a) and borehole SP gradient (Fig. 5c) provide a reasonable match to field observations (the residuals are shown in Fig. 5b). The model properties were varied iteratively to obtain this match. Most models fail to match the data. The total SP is anti-correlated with head (Fig.5(c)). • 𝑽𝑬𝑬𝑫 is the dominant SP source in the model and is in phase and anti-correlated with the change in head (Fig. 5(c)-(e)). 𝑽𝑬𝑬𝑫 and 𝑽𝑬𝑲 and are one time step out of phase. The 𝑽𝑬𝑬𝑫 is solely responsible for the borehole SP gradient and primarily responsible for the 12.42 hour tidal response. • The model results suggest that the borehole SP gradient is caused by the proximity of the saline front to the base of the borehole (c. 4m away in the model). The tidal SP response is primarily caused by tidally-driven lateral variations in the location of the front within the fracture zone. (b) (c) (a) (f)

Figure 2. The inland borehole monitoring array consists of 4 electrodes with a maximum spacing of 24m. (b) Location of the field sites (NERC). (c) The coastal site. The black line shows the model profile. (d) The coastal borehole monitoring array consists of 14 electrodes at 2m intervals, with a maximum spacing of 24m, and 3 probes measuring conductivity, temperature and pressure.

FIELD EXPERIMENT RESULTS – COASTAL AND INLAND SP COMPARISON • The borehole SP and SP periodicity (from power spectral density (PSD) estimates) from the inland (Fig.3(a)&(b)) and coastal (Fig.3(c)&(d)) sites are compared. Both sites show a 12.42 hour period (M2 tidal period) but the power is two orders of magnitude larger at the coastal site. The coastal site also shows a persistent and clear gradient in SP with depth which is not present at the inland site. (a) (b) (d) (c) Inland borehole Coastal borehole 12.42hrs

12.42hrs

Figure 3. (a) SP versus depth in the inland borehole. (b) and (c) Lomb-Scargle PSD of the inland and coastal borehole SP respectively. (d) SP versus depth in the coastal borehole.

(d)

(e)

Figure 5. (a) Model results for the total SP compared to field observations. (b) Model residuals showing an approximately normal distribution with a mean residual of 0.068mV, suggesting a good model fit to the observed data (c) Total SP and head in the model are anti-correlated, as observed in the field data (see Fig.4(b)). (d) 𝑽𝑬𝑬𝑫 contribution to the tidal SP. (e) 𝑽𝑬𝑲 contribution to the tidal SP. (f) Comparison of the model borehole gradient against the observed borehole gradient. This gradient is dominated by 𝑽𝑬𝑬𝑫 .

CONCLUSIONS • Significant semi-diurnal variations in borehole SP with a 12.42 hour period are observed in a coastal aquifer. The comparable response in an inland aquifer is two orders of magnitude smaller. A significant gradient in SP with depth is also observed in a coastal borehole, with no such gradient observed inland. • Numerical modelling suggests that the borehole SP response in the coastal aquifer is dominated by the exclusion-diffusion potential across a saline front that does not intersect the borehole • The SP gradient with depth is caused by the close proximity of the saline front to the base of the borehole. The tidal SP response is primarily caused by tidallydriven lateral variations in the location of the front within a sub-horizontal fracture zone. • Borehole SP monitoring can be used to monitor the location of a remote saline front in a coastal aquifer.