The impact of impoundment and urbanization on shallow groundwater conditions in the Joe Pool Lake catchment, Texas.

摘要

This dissertation is an assessment of the shallow groundwater (SGW) regime within the drainage area of Joe Pool Lake (JPL), an impoundment on Mountain Creek (MC) located in Grand Prairie, Texas. A thin zone of highly-weathered consolidated rock is normally found at the upper surface of the bedrock in north-central Texas; the SGW regime, or shallow aquifer, as defined for this study, extends vertically downward from the water table to the base of this highly-weathered layer, and extends laterally over the surface water drainage area of JPL and its tributaries. Geologically, the shallow aquifer thus consists largely of unconsolidated material, chiefly Holocene and Quaternary deposits on the floodplains and terraces of MC, Walnut Creek, and other creeks of the MC drainage system. The creation of JPL has, in itself, had a significant impact on the shallow groundwater regime; rapid urbanization has occurred in the vicinity of JPL since the completion of the impoundment, and questions exist as to the effects, current or potential, of this urbanization on the SGW. The objective of the research reported herein was to examine and analyze the impact of the lake itself on the SGW, and to evaluate the impacts of the subsequent urbanization on the hydrology of the JPL watershed.;The research utilized finite-difference simulation of groundwater flow, applied in an investigative sense, to gain an understanding of the hydraulics of the groundwater regime, and used Geographic Information System (GIS) technology to prepare simulation input data, to process and interpret supporting data, and to analyze and evaluate simulation results. The flow simulations were undertaken using the U.S. Geological Survey's MODFLOW-2005 software to calculate heads and model-wide flow totals, and using the MODFLOW postprocessor ZONEBUDGET-1996 to calculate internal flows and sub-regional groundwater budgets. Two steady-state simulations were performed with the objective of determining the rates and directions of groundwater (GW) flow throughout the SGW, in an average or long-term sense, first as the system now exists, with the lake in place, and then as the system existed prior to construction of the lake. In each simulation, it was assumed that the GW flow regime was in hydrologic equilibrium, and was limited by GW divides coinciding with the topographic divides surrounding the study area. The model was calibrated by comparing the observed borehole water level with calculated heads and comparing the baseflow per unit area of the catchment with the model calculated flow from SGW into the river. The optimum model thus obtained was again simulated using an unconfined approach so that the heads above the land surface and dry heads were kept within 25% of total. Finally the head distribution was found to be a replica of the subdued image of the topography.;Based on the results of simulation and the information supported by land-use, streamflow analysis, and demographic data, it appears that whereas the SGW in general is sensitive to the external systems and factors, to date the impact of impoundment and urbanization have been moderate. Obviously, in any natural setting the effect of recharge due to population-based water is trivial compared to the effect of land use change and urbanization; the impact of rainfall is utmost.;The presence of the lake has had an impact on the area hydrology and GW-SW interaction has changed. Prior to lake construction, GW discharged directly to the streams, and by near-surface discharge processes from the water table. In the present regime, GW discharge is primarily into the lake. From there discharge is by lake evaporation, gated discharge, municipal use, etc. The model results provide information that, the recharge to the GW regime has increased by about 5%, whereas the volume of SGW has increased by about 4%, although the distribution of GW has changed. Water levels are higher around the lake, but tend to be lower farther from the lake. The recharge from the shallow aquifer into the strata of the Woodbine is approximately similar in pre and post-lake conditions. Urbanization has imported extra water, but much of the import is either lost by near-surface discharge processes, or exported through sanitary sewer systems, or directly discharged to the SW system through storm sewer network. More work with well distributed observed water table data, improved versions of MODFLOW 2005 such as MODFLOW NWT and MODFLOW USG, and transient simulation would be required for an improvement of the model.