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Application of an analytic element model for multi-aquifer flow in the Atlantic coastal plain, USA
767
Application of an analytic element model for multi-aquifer flow in the Atlantic coastal plain, USA Stephen R. Kraemer a *, Mark Bakker, b aEcosystems Research Division, US Environmental Protection Agency, Athens, GA 30605, USA, [email protected] bDepartment of Agricultural and Biological Engineering, University of Georgia, Athens, GA 30602, USA, [email protected] 1. I N T R O D U C T I O N The poster presentation will demonstrate the application of a new analytic element program for multi-aquifer flow [1,2] to a shallow coastal plain aquifer system in Greene County, North Carolina, USA. The field site is under investigation to determine the impact of land spraying of liquid hog waste on subsurface and surface water quality. Excess nitrogen as nitrate is being measured underneath the field with a network of monitoring wells, and in the streams by taking discrete samples. Field-scale observations are being used to build watershed-scale models. The purpose of the simplified multi-layer modelling study is to test the hypothesis that simulations based on two homogeneous layers are adequate for understanding the overall water balance, and will form the basis for future estimates of nitrogen-nitrate loadings to streams in this watershed. Preliminary results will be presented. The study area is characterized by broad, somewhat-poorly to well-drained upland fiats 22-24 meters above mean sea level. The unconfined aquifer is composed of post-Pliocene fine to medium sand deposits extending to a depth of 6-9 meters beneath the upland fiats. The Yorktown confinining layer, 2-6 meters thick, separates the unconfined aquifer from the confined Yorktown aquifer. The Yorktown aquifer, 3-10 meters thick, overlies the Peedee confining unit, and is composed of fine to coarse shelley-sand. Mixed sand and clay alluvium is associated with the major streams drainages. The transition from upland fiat to alluvium is marked by terraces [4]. 2. M E T H O D S
The conceptual model of the aquifer flow system is simplified from the detailed stratigraphy model at the Lizzie field site [3]. The model has two layers beneath the upland *Notice: The U.S. Environmental Protection Agency through its Office of Research and Development partially funded and collaborated in the research described here under contract 1L-1216-NAEX to the University of Georgia, and under Interagency Agreement with the US Geological Survey, Raleigh, NC, DW14938792. It has been subjected to Agency review and approved for publication.
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Figure 1. Generalized cross section for two-layer model (not to scale). In the single aquifer zones: hydraulic conductivity k0 - 20 m/day, base elevation Zob= 1.7 m, top elevation Zot-- 20.2 m. In the two aquifer zone: aquitard resistance c = 10000 days, k l - 3 0 m/day, k2=20 m/day. Regional areal recharge N = 8.34E-4 m/day.
flats and a single layer in the alluvium. Average elevations of the strata are shown in Figure 1. Pumping tests were conducted by NCDENR to measure hydraulic conductivities. The aquitard resistance was estimated based on grain size analyses from cores taken by NCGS. A polygon inhomogeneity defining the two-layer upland zone was based on the geomorphology map of Greene County by NCDENR. A single circular element was superimposed covering the entire study area and given a uniform recharge rate of 8.34E-4 m/day (12 in/yr), which is the long term average. Constant-head linesinks were used to represent the perennial streams; the location and water elevation (head) was inferred from US Geological Survey topographic maps (1:24000 scale) and the US Dept. of Agriculture soil survey maps for Greene County. 3. P R E L I M I N A R Y
RESULTS AND DISCUSSION
Contours of head for the study region are shown in Figure 2. The heads predicted by the model were compared to observed heads in NCDENR monitoring wells at the Lizzie site. The comparison is made to three well locations screened in the the top aquifer (L15,L24,L27) and screened in the bottom aquifer (L14,L22,L26). Water levels were measured during three surveys (llAug99,20Sep99,19Jan00). The mean absolute error for the simulation presented here is 1.8m for the top aquifer, and 0.8m for the bottom aquifer, but the model has not been calibrated extensively. See Figure 3. Work continues to characterize ground water/surface water interactions (e.g., water balances and nitrate loadings) using analytic element models at the Lizzie site. Future code developments and applications may include transient elements, three-dimensional tracing, and polygonal areasinks.
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Figure 2. Regional head contours (meters above mean sea level). Shaded area represents polygon inhomogeneity; outside is one aquifer, inside are two aquifers separated by aquitard. Solid line contours are in the top aquifer. Dotted line contours are in the bottom aquifer.
The computer program demonstrated above has been implemented in Python (w~ru.python, org) and the code is available under an Open Source license at ww~. engr. uga. edu/~mbakker/t •
html.
REFERENCES
1. M. Bakker, Analytic elements for the modeling of multi-aquifer flow, Proceedings Computational Methods Water Resources, Delft, Netherlands (2002). 2. M. Bakker and O.D.L. Strack, Analytic elements for multi-aquifer flow, submitted to Journal of Hydrology, (2002). 3. K. Farrell, NC Geological Survey (w,-~.geology.enr.state.nc.us/Default.htm), T. Mew, NC Dept. Environment and Natural Resources, personal communication, 2002.
4. T. Mew, and T. Spruill, Lizzie Research Station, NC Dept. Environment and Natural Resources, Division of Water Quality, Groundwater Section, Raleigh, NC, 4p., (2000).
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Figure 3. Lizzie site head contours for the top and bottom aquifers. The error at the observation points are shown as proportionally-scaled triangles; point up for model prediction greater than observed; point down for model prediction less than observed. The value of the error is placed above the symbol.
Application of an analytic element model for multi-aquifer flow in the Atlantic coastal plain, USA Stephen R. Kraemer a *, Mark Bakker, b aEcosystems Research Division, US Environmental Protection Agency, Athens, GA 30605, USA, [email protected] bDepartment of Agricultural and Biological Engineering, University of Georgia, Athens, GA 30602, USA, [email protected] 1. I N T R O D U C T I O N The poster presentation will demonstrate the application of a new analytic element program for multi-aquifer flow [1,2] to a shallow coastal plain aquifer system in Greene County, North Carolina, USA. The field site is under investigation to determine the impact of land spraying of liquid hog waste on subsurface and surface water quality. Excess nitrogen as nitrate is being measured underneath the field with a network of monitoring wells, and in the streams by taking discrete samples. Field-scale observations are being used to build watershed-scale models. The purpose of the simplified multi-layer modelling study is to test the hypothesis that simulations based on two homogeneous layers are adequate for understanding the overall water balance, and will form the basis for future estimates of nitrogen-nitrate loadings to streams in this watershed. Preliminary results will be presented. The study area is characterized by broad, somewhat-poorly to well-drained upland fiats 22-24 meters above mean sea level. The unconfined aquifer is composed of post-Pliocene fine to medium sand deposits extending to a depth of 6-9 meters beneath the upland fiats. The Yorktown confinining layer, 2-6 meters thick, separates the unconfined aquifer from the confined Yorktown aquifer. The Yorktown aquifer, 3-10 meters thick, overlies the Peedee confining unit, and is composed of fine to coarse shelley-sand. Mixed sand and clay alluvium is associated with the major streams drainages. The transition from upland fiat to alluvium is marked by terraces [4]. 2. M E T H O D S
The conceptual model of the aquifer flow system is simplified from the detailed stratigraphy model at the Lizzie field site [3]. The model has two layers beneath the upland *Notice: The U.S. Environmental Protection Agency through its Office of Research and Development partially funded and collaborated in the research described here under contract 1L-1216-NAEX to the University of Georgia, and under Interagency Agreement with the US Geological Survey, Raleigh, NC, DW14938792. It has been subjected to Agency review and approved for publication.
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Figure 1. Generalized cross section for two-layer model (not to scale). In the single aquifer zones: hydraulic conductivity k0 - 20 m/day, base elevation Zob= 1.7 m, top elevation Zot-- 20.2 m. In the two aquifer zone: aquitard resistance c = 10000 days, k l - 3 0 m/day, k2=20 m/day. Regional areal recharge N = 8.34E-4 m/day.
flats and a single layer in the alluvium. Average elevations of the strata are shown in Figure 1. Pumping tests were conducted by NCDENR to measure hydraulic conductivities. The aquitard resistance was estimated based on grain size analyses from cores taken by NCGS. A polygon inhomogeneity defining the two-layer upland zone was based on the geomorphology map of Greene County by NCDENR. A single circular element was superimposed covering the entire study area and given a uniform recharge rate of 8.34E-4 m/day (12 in/yr), which is the long term average. Constant-head linesinks were used to represent the perennial streams; the location and water elevation (head) was inferred from US Geological Survey topographic maps (1:24000 scale) and the US Dept. of Agriculture soil survey maps for Greene County. 3. P R E L I M I N A R Y
RESULTS AND DISCUSSION
Contours of head for the study region are shown in Figure 2. The heads predicted by the model were compared to observed heads in NCDENR monitoring wells at the Lizzie site. The comparison is made to three well locations screened in the the top aquifer (L15,L24,L27) and screened in the bottom aquifer (L14,L22,L26). Water levels were measured during three surveys (llAug99,20Sep99,19Jan00). The mean absolute error for the simulation presented here is 1.8m for the top aquifer, and 0.8m for the bottom aquifer, but the model has not been calibrated extensively. See Figure 3. Work continues to characterize ground water/surface water interactions (e.g., water balances and nitrate loadings) using analytic element models at the Lizzie site. Future code developments and applications may include transient elements, three-dimensional tracing, and polygonal areasinks.
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1655000 m
A'
Figure 2. Regional head contours (meters above mean sea level). Shaded area represents polygon inhomogeneity; outside is one aquifer, inside are two aquifers separated by aquitard. Solid line contours are in the top aquifer. Dotted line contours are in the bottom aquifer.
The computer program demonstrated above has been implemented in Python (w~ru.python, org) and the code is available under an Open Source license at ww~. engr. uga. edu/~mbakker/t •
html.
REFERENCES
1. M. Bakker, Analytic elements for the modeling of multi-aquifer flow, Proceedings Computational Methods Water Resources, Delft, Netherlands (2002). 2. M. Bakker and O.D.L. Strack, Analytic elements for multi-aquifer flow, submitted to Journal of Hydrology, (2002). 3. K. Farrell, NC Geological Survey (w,-~.geology.enr.state.nc.us/Default.htm), T. Mew, NC Dept. Environment and Natural Resources, personal communication, 2002.
4. T. Mew, and T. Spruill, Lizzie Research Station, NC Dept. Environment and Natural Resources, Division of Water Quality, Groundwater Section, Raleigh, NC, 4p., (2000).
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Figure 3. Lizzie site head contours for the top and bottom aquifers. The error at the observation points are shown as proportionally-scaled triangles; point up for model prediction greater than observed; point down for model prediction less than observed. The value of the error is placed above the symbol.