Suspended Sediment Concentrations Downstream of a Harvested ...

Suspended Sediment Concentrations Downstream of a Harvested Peat Bog: Analysis and Preliminary Modelling of Exceedances Using Logistic Regression André St-Hilaire, Simon C. Courtenay, Carlos Diaz-Delgado, Bronwyn Pavey, Taha B.M.J. Ouarda, Andrew Boghen and Bernard Bobée

Abstract: Acting as natural filters, peatlands are important wetland ecosystems in many northern countries, including Canada. To harvest peat, the vegetation must be removed and the harvested area ditched to drain and dry the peat. Drainage ditches are often designed to route water to settling ponds prior to releasing runoff into nearby water bodies. The present study investigated one key water quality variable, suspended sediment concentration (SSC), downstream of settling ponds in an actively harvested peatland. Time series of SSC for two spring seasons (2001-2002) were recorded at two sites using optical back scatterometers (OBS) calibrated in situ. SSC values exceeded the New Brunswick provincial guideline of 25 mg/L between 53.6 and 86.0% of the time. Even when the threshold was raised to relatively high values such as 500 mg/L, the percentage of exceedance remained relatively high (between 11 and 60%). A statistical model of SSC exceedance, based on logistic regression, was tested to investigate which hydrological forcings may explain high SSC values. Various independent variables were used in conjunction with an autoregressive component and were compared using different goodness of fit criteria. For a threshold of 500 mg/L, the best fit among all the logistic regression models tested included lag 1 and 2 autoregressive terms, as well as five-day cumulative precipitation, air temperature and three-day lagged discharge. The model was able to correctly predict 82% of exceedances. Résumé : Les tourbières sont des écosystèmes de première importance dans de nombreux pays nordiques, y compris le Canada. La récolte de tourbe nécessite la dénudation de la surface exploitée, suivie par la mise en place d’un réseau de canaux de drainage afin d’assécher la couche superficielle de tourbe. Pour minimiser l’impact de ces particules en suspension, les canaux de drainage sont dirigés vers des bassins de sédimentation. Deux sites ont été échantillonnés durant la période printanière de 2001 et 2002 à l’aide de néphélomètres (OBS) ayant été calibrés in situ. Les séries chronologiques de concentration de solides en suspension (CSS) ainsi obtenues ont été converties en séries dichotomiques de dépassement André St-Hilaire1, Simon C. Courtenay2,4, Carlos Diaz-Delgado3, Bronwyn Pavey1, Taha B.M.J. Ouarda1, Andrew Boghen4 and Bernard Bobée1 INRS-ETE, Quebec City, QC G1K 9A9 Fisheries and Oceans Canada at the Canadian Rivers Institute, Department Biology, University of New Brunswick, Fredericton, NB E3B 6E1 3 Centro Interamericano de Recursos del Agua, Facultad de Ingeniería, Universidad Autónoma del Estado de México. C.U. s/n, Toluca, Estado de México, C.P. 50130 4 University of Moncton, Faculty of Graduate Studies and Biology Department, Moncton, NB E1A 3E9 1 2

Submitted December 2005; accepted April 2006. Written comments on this paper will be accepted until March 2007. Canadian Water Resources Journal Revue canadienne des ressources hydriques

Vol. 31(3): 139–156 (2006)

© 2006 Canadian Water Resources Association

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Canadian Water Resources Journal/Revue canadienne des ressources hydriques

et non dépassement de certains seuils de CSS. Les résultats ont démontré que durant les deux printemps échantillonnés, les valeurs de CSS ont dépassé la norme provinciale de 25 mg/L entre 53.6% et 86% du temps. Même si on élève le seuil à une valeur de SSC plus importante, tel que 500 mg/L, le pourcentage des dépassements demeure élevé (entre 11% et 60%). Une étude préliminaire sur la faisabilité de l’utilisation de la régression logistique pour modéliser les dépassements a été complétée. Pour un seuil de 500 mg/L, le meilleur modèle inclut des termes autorégressifs d’ordre 1 et 2, de même que la température de l’air, le cumul de précipitations de 5 jours et le débit décalé de 3 jours. Le modèle a permis de prédire correctement 82% des dépassements.

Introduction Peatlands are important ecosystems in Canada. These ecosystems cover 113 million hectares, corresponding to 11% of the country’s land area and nearly 75% of all Canadian wetlands. Canada harvests approximately five percent of the world volume of peat and is the most important producer of horticultural peat in the world. The province of New Brunswick produces 36% of Canadian peat, with a value of $95 million (2003). The hydrology of ombrotrophic (or raised) bogs is dominated by atmospheric precipitation since such surfaces are usually isolated from groundwater inflows (Reeve et al., 1996). In actively growing peatlands, the rate of production of organic material exceeds the rate of degradation, leading to an accumulation of organic matter. Water storage within peat is a major factor contributing to variations in the vertical movement of water in peatlands. Undisturbed peatland catchments are often characterized by saturated soils and low infiltration rates, which can lead to high storm runoff (Holden and Burt, 2002). The infiltration capacity of a peat blanket varies as a function of compression and swelling of peat (Holden and Burt, 2003). Any major perturbation of these ecosystems may modify their hydrological properties. Peat harvesting necessitates the removal of all vegetation from the harvested surface. In addition, a dense network of drainage ditches is required to lower the water content of the mire, thereby allowing the upper

layer of peat to dry and be harvested using industrial vacuums (the predominant method of harvesting in Canada). The drainage system in harvested peatlands can reduce the water table level, which can lead to greater infiltration capacity. Kløve (1998) noted that in drained peatlands, a substantial amount of rainfall is needed to produce overland flow. However, Price et al. (2005) explained that the changes in peat structure and hydraulic conductivity associated with harvesting limit the effective lowering of the water table. In some cases, overland flow in harvested peatlands has been found to increase substantially during low intensity precipitation events. The subsequent changes in the hydrological regime in harvested peatlands result from a combination of factors, including: 1) decreased evapotranspiration associated with the removal of the vegetation cover; 2) depending on the drain network, faster routing of overland flow via the ditches; and 3) changes in the physical structure of dried peat leading to hydrophobicity (Gregory et al., 1984; Van Seters and Price, 2002). The quality of water flowing out of harvested peatlands is variable. Generally, peat extraction has been associated with relatively low pH values and high phosphorus and nitrate loadings (Olsson, 1985; Kløve, 1998; Surette et al., 2002; St-Hilaire et al., 2004). Harvested peatlands have also been found to alter water quality through a substantial increase in suspended sediment concentrations and loads caused by erosion of peat from the mine surface and ditch channels (Kløve, 1998). To mitigate a potential water quality decline downstream of harvested peatlands, many regulatory agencies, including the New Brunswick Department of Natural Resources, recommend settling ponds be installed immediately downstream of the drainage network and upstream of the receiving stream, estuary or coastal area. Settling ponds are structured so that velocity decreases as drainage water passes through the pond, allowing suspended sediments to drop out of the water column. New Brunswick provincial guidelines stipulate that suspended sediment concentrations downstream of the ponds should not exceed 25 mg/L (Thibault, 2001). Pond volume to achieve such concentrations depends on the size of area being harvested and is suggested to follow the ratio of 25 m3/ha. Few studies have investigated the efficiency of settling ponds. Joensuu (2002) examined the efficiency of a settling pond downstream of a network of drainage ditches in Finnish bogs and estimated that © 2006 Canadian Water Resources Association

St-Hilaire, Courtenay, Diaz-Delgado, Pavey, Ouarda, Boghen and Bobée

the ponds reduced suspended sediment concentrations by 28%. Kløve (1998) measured sediment retention in ponds during controlled precipitation experiments and compared three methods of sediment retention at an experimental site in Finland. He found that the inclusion of a buffer zone and a flood control structure reduced the suspended sediment concentration by 50% during the spring snowmelt, compared to a stand alone settling pond (Kløve, 2000). To date, there has been only one study specifically concerned with the characteristics and efficiency of settling ponds found in Canadian harvested peatlands. GEMTEC (1993) reviewed the characteristics of existing designs worldwide and compared them to the results of a pilot study in New Brunswick. They found that increasing basin volume may not be a sufficient mitigation measure. Basins for which the pond volume to harvested area ratio exceeded 25 m3/ha did not always retain sediments efficiently. In a more general peatland runoff study at a site located near Acadieville, in southern New Brunswick, GEMTEC (1994) found that, on average, the settling pond reduced suspended solid concentrations (SSC) by 23%, but that the mean summer concentration was 32 mg/L, which is higher than the New Brunswick guideline. Since the efficiency of settling ponds is variable, there is a need to better understand the causes and timing of high SSC events downstream of harvested peatlands equipped with such ponds. The challenge is made more difficult by limited hydrological data near harvested sites and even fewer time series of SSC collected downstream of harvested peatlands. This paucity of quantitative data makes it difficult to use traditional Ordinary Least Squares (OLS) regression models implemented on other types of drainage basins (e.g., Bray and Huixi, 1993; Richards and Moore, 2003) to investigate the causes of high SSC events. The primary purpose of this study is to test a statistical model for exceedance of a SSC threshold downstream of a harvested peatland. The case study is located in the St. Charles Plain near Rexton, New Brunswick. Harvesting was initiated in 1985. During the first nine years, drainage water from this peatland flowed directly in an unnamed brook (hereafter Malpec Brook). Three settling ponds were constructed in 1994 to retain peat particles and other suspended sediments after peat was found to be accumulating in Mill Creek. A peat delta covering an area of 0.9-1.0 ha

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can be observed at low tide at the confluence of Malpec Brook and Mill Creek (MGI Limited, 1994; Ouellette et al., 2006). Surveys of peat depth and areal coverage in Mill Creek have revealed that peat depth increases ranged from six cm to 31 cm and the percentage of total volume of sediments containing peat in the top 15 cm of the substrate increased from 35% in 1997, to 44% in 1998 and 76% in 1999 (Ouellette et al., 2003; 2006). These findings triggered the present study. The specific objectives of this study are: 1. To document the chronology of suspended sediment concentrations (SSC) during the spring season, which is characterized by high hydrologic variability; 2. To record the frequency of exceedance of the New Brunswick guideline for maximum SSC; and 3. To test the use of logistic regression to model SSC exceedances in a peatland environment. To achieve these objectives, SSC thresholds must be defined. The threshold exceedance of 25 mg/L is of interest in this project because it is the current New Brunswick guideline. Higher thresholds were also examined because they have been linked in the literature to adverse effects on aquatic biota (St-Hilaire et al., 1997). Auld and Schubel (1978) mentioned that concentrations of suspended sediments greater than 100 mg/L reduced the hatching success of white perch (Morone americana) and striped bass (Morone saxatilis), while SSC>500 mg/L significantly reduced larval survival. The Richibucto drainage basin is a spawning and nursery area for white perch (St-Hilaire et al., 2002) and young of the year striped bass spawned in the nearby Miramichi River rear and overwinter in the Richibucto River and its tributaries (Robinson et al., 2004). Logistic regression has been used in a number of recent studies, including Milot et al. (2000) who estimated the probability of threshold exceedances of drinking water standards for toxic by-products. The same approach has been used in toxicological studies of sediments (e.g., Field et al., 1999) as well as to model water quality guideline exceedances in an estuarine impoundment (Worrall et al., 1998). © 2006 Canadian Water Resources Association

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Study Area and Sampling Protocol Study Area

Sampling of SSC was performed at two sites located downstream of the harvested portion of the St. Charles Plain (Figure 1), which is a peatland spanning 19.3 km2. The total drainage area of the harvested portion of the peatland is approximately four km2. Suspended sediment concentrations were monitored during the spring of 2001, as well as the spring and fall of 2002. Malpec Brook flows into Mill Creek, an estuarine tributary of the Richibucto River (Figure 1). Although Mill Creek is tidal, Malpec Brook stations are located upstream, at higher elevations, and are not subjected to backwater effects. Climate normals for the area were computed from data gathered at the Rexton meteorological station, located less than five km from the study site (station 8104400, latitude 45°40’N, longitude 64°52’W; Environment Canada, CDCD East database http://

www.climate.weatheroffice.ec.gc.ca). Mean (19222000) annual air temperature is 5°C. Daily temperature in March varies between -8.9 and 1.7°C, with an interannual mean of -3.6°C. In April, the minimum, mean and maximum temperatures are -2.2, 2.6 and 7.4°C, respectively. In May, the same climate normals are 3.8, 9.5 and 15.2°C, respectively. Mean (65 years) total annual precipitation in the area is 817.5 mm, 29% of which is snow. The maximum recorded total annual precipitation was 1189 mm. Two sites were selected. The upstream site was located in the main outflow ditch, which routes all of the drainage water coming from the ponds into Malpec Brook. The downstream site was located in Malpec Brook, approximately halfway between the peat operation and Mill Creek (Figure 1). Both sites are characterized by a relatively narrow channel ( 500 mg/L, transition counts drop significantly after lag two at both sites, but more so at the upstream site. This is an indication that the autoregressive component of order two in the proposed logistic regression model is adequate for this particular binary time series, whereas additional autoregressive terms would be required for Table 2. Transition counts for consecutive exceedances (P11) calculated from time series of spring 2001 at both sites and for three thresholds: 25 mg/L, 100 mg/L and 500 mg/L. The section in bold corresponds to the selected threshold used in the logistic regression. Upstream

Downstream

Threshold = 100 mg/L

Threshold = 25 mg/L

2001

2002

2001

2002

P11

P11

P11

P11

0.74 0.67 0.69 0.66

0.95 0.90 0.88 0.86

0.90 0.80 0.73 0.68

0.88 0.80 0.75 0.74

Threshold = 100 mg/L 2001 P11

0.77 0.62 0.55 0.50

2002 P11

0.89 0.81 0.76 0.73


Threshold = 100 mg/L 2001 P11

0.77 0.52 0.33 0.21

2002

2001

P11

P11

P11

0.77 0.63 0.63 0.57

P11

0.86 0.77 0.71 0.68


2001

0.73 0.45 0.27 0.18

2002

0.50 0.20 0.10 0.00

2002 P11

0.78 0.57 0.48 0.43

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lower thresholds. If lower thresholds were used in Equation (3), the high order autoregressive component of the model may overshadow the influence of exogenous variables. To avoid such a possibility, the upstream site was used as a test case and 500 mg/L was selected as the threshold to be exceeded to define “successes” (1), while values 500 mg/L) with goodness of fit statistics. Model #

1 2 3 4 5

6

Independent variables

SSC > 500 Lag 1 SSC > 500 Lag 2

-2LLF

Chi-square

P-value

r 2p

Specificity

Sensitivity

(%correct 0)

(% correct 1)

29.14

24.29

0.0001

0.57

97.6

72.7

SSC > 500 Lag 1 SSC > 500 Lag 2

29.90

41.83

0.00014

0.57

97.6

72.7

SSC > 500 Lag 1 SSC > 500 Lag 2

17.96

36.17

0.000001

0.77

95.2

72.7

SSC > 500 Lag 1 SSC > 500 Lag 2

15.15

38.99

500 Lag 2

5-Day Cumm. Precip Flow Lag 3

Air temperature

5-Day Cumm. Precip Temperature

SSC > 500 Lag 1 SSC > 500 Lag 2

5-Day Cumm. Precip

25.78

28.35

0.00001

0.65

97.6

72.7

Temperature Flow Lag 3

Figure 6. Lagged cross correlation between SSC at the downstream site and discharge data.



season in the study region. Inclusion of air temperature as the sole independent variable in addition to the autoregressive terms was found to provide a significant model improvement (-2LLF = 17.96, r 2p = 0.77; Table 3), better than the improvement obtained by using precipitation. Finally, multivariate models were tested. Model #5 in Table 3 includes both cumulative precipitation and air temperature. An increase in r 2p (0.81) and an increase in sensitivity (81.8%) are observed for this model. The inclusion of all three independent variables (Model #6) increases the value of r 2p to 0.85, compared to 0.57 for a strictly autoregressive model. The best model therefore includes all three independent variables. When compared with a purely autoregressive model, Model #6 provides an improvement in sensitivity of 9.1% (i.e., from 72.7 to 81.8%), while specificity remains constant at 97.6%.

Discussion Times series of SSC gathered during the spring of 2001 and 2002 in Malpec Brook showed mean daily SSC higher than the New Brunswick provincial guideline for a majority of the sampled days. These findings contradict those of Kløve (1998) who found that sediment concentrations remained low during snowmelt events. However, his work did not focus specifically on snowmelt, and his SSC sampling frequency was lower than in the present study. High SSC events may therefore have been missed in the study of Kløve (1998), which could explain these differences. Holden and Burt (2002) reported SSC during short field rainfall simulation experiments on small unvegetated peatland plots. They monitored SSC during a period of 150 hours with a rainfall intensity varying between three and 12 mm/h. SSC varied between 33 and 3852 mg/L. The range and upper limit reported by Holden and Burt (2002) are of the same order of magnitude as those measured in this study. As no SSC monitoring was performed upstream of the ponds, the present results cannot be used to directly assess the capability of the ponds to attenuate suspended sediment loads. However, it is clear that the SSC downstream of the ponds do not meet present guidelines. Interannual variability can be partially explained by hydro-meteorological factors. Although mean discharge for April/June was higher in 2001 (13.85 m3/s at station 01BS001) than in 2002 (8.06 m3/s), the month of April

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was colder in 2001 (mean temperature of 2.8°C) than in 2002 (4.2°C), which means that the spring snowmelt occurred later in the spring of 2001 than in 2002. In fact, the field surveys have shown that a considerable volume of snow was still on the ground as late as May 25, 2001. Hence it is possible that the higher turbidity events during the spring season of 2001, which may be associated with spring snowmelt, occurred after the sampling period. Our sampling period was better synchronized with the spring flood in 2002. Another important factor that may have increased mean turbidity in 2002, particularly at the upstream station, was the malfunctioning of one settling pond during the same year. The pond was only repaired during the fall of 2002. It is also known from field observations that the ponds were still partly frozen during high spring runoff events. It is therefore possible that surface water charged with suspended sediments simply by-passed the ponds altogether. A number of agencies now use maximum concentrations and maximum total daily loads to regulate or set guidelines for the safekeeping of aquatic habitats. Even with the limited data available for this study, the logistic regression proved to be useful in determining which of the variables may act as triggers for threshold exceedances. The fact that dichotomic variables can be included in the model offers promising possibilities such as the inclusion of independent qualitative variables that describe operational constraints from the industry (e.g., harvesting versus non-harvesting, pond maintenance, ditching, etc.). Two major constraints were found when using the logistic regression in this preliminary study: 1) the autocorrelation structure of the binary time series shows strong persistence for lower thresholds. This is associated with the fact that SSC were measured at high frequency for this study. In most operational cases, SSC values are not taken at a daily time step (weekly or monthly measurements are more common). Therefore it is possible that SSC measurements taken by the producers may be considered independent and that the autoregressive component of the model may not be required. However, a model developed using sparse data will likely provide less information. 2) Local discharge time series were not available. The best discharge data available for use in this study originated from a larger, more forested subcatchment of the Richibucto River. Although this may be seen as an important gap in the data required for a detailed © 2006 Canadian Water Resources Association

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analysis and the development of a more robust model, it must be noted that most harvested peatlands in Canada are located on ungauged basins. Models that can use alternative independent variables must be developed. For spring events, our preliminary results show that air temperature appears to be worth investigating. In looking for alternative independent variables, care should be given to selecting variables that minimize colinearity. In the specific application shown in this study, correlation between independent variables remained relatively low (r < 0.17). Local discharge measurements would allow for a more precise estimation of the proper lag required to model SSC using discharge data. This preliminary investigation indicates that a lag of the order of three days is necessary to properly model SSC in Malpec Brook. It is possible that this lag applies only to this case and may be an artefact caused by the source of discharge data (i.e., a different sub-catchment). It is also possible that harvested peatlands may be characterised by important lags between flow and SSC peaks. More data will be required before such a conclusion can be confirmed. It is likely that the water retention capability of such mires varies with harvesting operations, which change the nature of soil properties, such as hydraulic conductivity. The SSC established in this study are often in excess of those suitable for aquatic biota. Studies by Ouellette et al. (1997; 2003; 2006) have shown that peat accumulation downstream of Malpec Brook has had adverse effects on epibenthic fauna. The US Environmental Protection Agency (USEPA) classifies sediment as the leading cause of water quality impairment of assessed rivers and lakes (Gray et al., 2000). In this study, the high SSC events identified during both spring seasons often lasted for several days. These long periods of very high turbidity may contribute to heightened stress levels for various biota, including filter feeding molluscs, crustaceans and finfish that live in the nearby estuary. For instance, the Richibucto Estuary is home to the most important commercial aquaculture operation of American oyster (Crassostrea virginica) in the Maritime provinces of Canada. Oysters can survive in a very turbid environment, but prefer a SSC range between six mg/L and 700 mg/L (Shumway, 1996). That upper bound was exceeded on numerous occasions during the field sampling periods, especially in 2002.

These high SSC events have also translated into high suspended sediment loads in Malpec Brook and the associated estuary, where peat is known to accumulate (Ouellette et al., 2006). For instance, simply multiplying the mean SSC by the water volume associated with the estimated mean daily discharge in Malpec Brook provides a rough, conservative estimate of the sediment load. For the relatively short (68 days) spring sampling season of 2001, this yields an estimated load of 9.25 tonnes indicating that corrective measures are needed to increase the efficiency of the settling ponds at this site.

Conclusions High frequency monitoring of turbidity downstream of harvested peatland has shown that the current management practices of drainage water may be suboptimal for the spring season. Based on measurements taken during two spring seasons, the SSC limit suggested by the provincial regulatory agencies was exceeded more than 50% of the time in 2001 and more than 80% of the time in 2002. Preliminary tests on the applicability of logistic regression to select independent variables and to model threshold exceedances show that it may be a promising tool. The fact that the best model included three exogenous variables is an indication that the causes of high SSC events are probably numerous. This study is limited in scope (i.e., two sites on the same peatland, two field seasons). Future studies, including monitoring stations upstream of settling ponds and additional harvested sites and a control non-harvested bog, are needed to validate these findings. Furthermore, the majority of peat harvesting operations occur during the summer period during which convective storms constitute the most important hydrological events. The efficiency of settling ponds during such events should also be investigated.

Acknowledgements The authors wish to thank C. Calder and D. Robertson who did most of the field work for this project. The contributions of M. Boudreau, C. Ouellette and C.I. Bénaoudia are also acknowledged. Some of the initial descriptive statistical analyses were performed by © 2006 Canadian Water Resources Association


L. Benyahya to whom the authors are indebted. This work was funded by Premier Horticulture Ltd., DFO, NSERC, the Québec-New Brunswick Cooperation Agreement, the International Atomic Agency and the Canadian Bureau for International Education (U.N. Scholarship Mex103046P, Projects #MEX/1/022 and UAEM 1780/2003).

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