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SUBSURFACE RESIDENCE TIMES AS AN ALGORITHM FOR AQUIFER SENSITIVITY MAPPING: TESTING THE CONCEPT WITH GROUND WATER MODELS IN THE CONTENTNEA CREEK BASIN, NORTH CAROLINA, USA
Kraemer, S R. SUBSURFACE RESIDENCE TIMES AS AN ALGORITHM FOR AQUIFER SENSITIVITY MAPPING: TESTING THE CONCEPT WITH GROUND WATER MODELS IN THE CONTENTNEA CREEK BASIN, NORTH CAROLINA, USA. Presented at MODFLOW 2001 and Other Modeling Odysseys Conference, Golden, CO, September 11-14, 2001.
Develop, test, and refine models to evaluate sub-basins to determine whether local water quality problems due to excessive nutrient loading exist, and if so, to characterize them and determine their relationships to nutrient loading. Develop models to simulate overland flow and non-point source pollutant loads to track and assess nutrient loadings across watersheds and provide approaches for estimating nutrient budgets within sub-basins and for predicting changes in nutrient budgets in response to changes in watershed activities/land use/land cover. Demonstrate the application of the recommended approach/models for predicting changes in nutrient budgets in response to changes in proposed watershed activities/land use/land cover, resulting in specific recommendations for reducing the nutrient loads to a basin. For coherence, cooperation, and economics, these models will be housed in a unified, consistent, computational environment for environmental analyses that allows teaching (i.e., technology transfer) to multiple users (users concentrate on problem, not model input/output); that appeals to multi-disciplinary groups for distribution and use as a consistent assessment methodology (includes models, tools, modular design and facilitated updates of science/engineering); that includes resident visualization, animation tools, documentation and tutorials on-line, hooks to GIS and environmental databases; and is executable on UNIX, personal computers, and HPC resources.
Objective # 2.2 Conserve and enhance nation's waters: By 2005, conserve and enhance the ecological health of the nation's (state, interstate, and tribal) waters and aquatic ecosystems-rivers and streams, lakes, wetlands, estuaries, coastal areas, oceans, and groundwater-so that 75% of waters will support healthy aquatic communities.
This poster will present a modeling and mapping assessment of landscape sensitivity to non-point source pollution as applied to a hierarchy of catchment drainages in the Coastal Plain of the state of North Carolina. Analysis of the subsurface residence time of water in shallow aquifers is useful for assessing the impact of fair weather loadings of non-point source pollution to streams, such as the drainage of nitrogen from agricultural fields via baseflow. While accurate prediction of pollution transport and transformation continues to challenge the most advanced methods of site characterization and computer modeling, the theoretical residence time distribution in shallow "groundwatersheds" offers the possibility of integrating complex heterogeneity into a relatively simple function.
The key determinants of the robustness of simple residence time functions (i.e., recharge rate N, porosity n, saturated thickness H) are scale dependent in representation. We use GIS and ground water flow models to test and assess the influence of scale and heterogeneity on subsurface residence times. The hierarchical scales of investigation range from site scale (Lizzie,NC, 1.5 km2), to subwatershed scale (Sandy Run/Middle Swamp, 14-digit HUC, 125 km2), to watershed scale (Little Contentnea Creek, 11-digit HUC, 525 km2) to basin scale (Contentnea Creek, 8-digit HUC, 2579 km2). The Lizzie site provides an opportunity to test simplified geologic conceptual models against observed complexity. A back-of-the-envelope analysis using the comprehensive potential builds understanding of the flow between the surficial aquifer, the confined aquifer, and the stream alluvium. The finite difference model MODFLOW and the analytic element model GFLOW are used to solve for the two-dimensional flow field and define the groundwatershed at the Sandy Run/Middle Swamp scale. The release of a grid of tracelines and their collection at the catchment outlet provides the quantification of the relative cumulative residence time distribution F(T). This distribution can be approximated by F(T) = 1 - exp (-T/T), where T = nH/N, provided the aquifer is piece-wise constant in the properties making T , which is the average subsurface residence time. The Arcview GIS system is used to extrapolate the spatial distribution of average residence times for the Contentnea Creek basin. The effective zoning of residence times is presented as an aquifer sensitivity map of streams to fair weather loadings of non-point source pollution.
The techniques presented are challenged by uncertainties in conceptual model and parameterization. Research continues on parameter estimation and quantifying uncertainty.