Simulation of Projected Water
Demand and Ground-Water Levels in the Coffee Sand and Eutaw-McShan Aquifers
in Union County, Mississippi, 2010 through 2050
SUMMARY
Ground water from the Eutaw-McShan and Coffee Sand aquifers, with lesser amounts withdrawn from the Gordo and Ripley aquifers, is the sole source of supply for residential, commercial, and industrial purposes in Union County, Mississippi. The recent accelerated rate of growth of the population and of the economy in Union County suggests that the need for additional ground water will increase in the future. Long-term projections are needed to determine if the aquifers can supply anticipated future municipal and nonmunicipal water demands for the water-service area to the year 2050.
Detailed water-use data and ancillary information for residential, commercial, and industrial sectors were collected for the 12 public-supply facilities and for the self-supplied commercial and industrial facilities in Union County for 1998. The data were used to document water use and to construct the linear-predictive and constant-rate models contained within Forecast Manager of the IWR-MAIN Water-Demand Management Suite software. Water demand to the year 2050 was estimated by relating water use to housing and employee counts, housing and employee types, median household income, marginal price of water, water-conservation practices, and long-term temperature and precipitation data.
In 1998, total ground-water withdrawals were estimated as 2.85 Mgal/d. Of that amount, municipal withdrawals were 2.55 Mgal/d. Residential deliveries from public-supply systems accounted for 59 percent (1.50 Mgal/d) of the municipal water; commercial and industrial, 18 percent (0.456 Mgal/d); and public/unaccounted water, about 23 percent (0.594 Mgal/d). Nonmunicipal withdrawals were 0.296 Mgal/d. About 80 percent (2.27 Mgal/d) of the water is pumped from the Eutaw-McShan aquifer; about 13 percent (0.371 Mgal/d) from the Coffee Sand aquifer; about 4 percent (0.129 Mgal/d) from the Gordo aquifer; and 3 percent (0.082 Mgal/d) from the Ripley aquifer.
Simulations of water demand were made using a normal- and a high-growth scenario. In a normal-growth scenario, total water demand would increase 72 percent from 2.9 Mgal/d in year 1998 to 5.0 Mgal/d in year 2050. Municipal demand would increase from 2.6 Mgal/d to 4.6 Mgal/d, or 77 percent. In a high growth-scenario, total water demand would increase 131 percent from 2.9 Mgal/d in year 1998 to 6.7 Mgal/d in year 2050. Municipal water demand would increase 146 percent during that same period. The rate of nonmunicipal use (0.30 Mgal/d) was held constant for the forecast years for both scenarios.
Simulations of projected ground-water levels were made using baseline-, normal-, and high-growth water demands. The ground-water model was constructed using the U.S. Geological Survey finite-difference computer code MODFLOW. The model had been previously calibrated as part of an earlier (1998) study of the aquifers comprising formations of Cretaceous and Paleozoic age in northeastern Mississippi. The calibrated ground-water flow model used in that investigation encompassed the entire area originally simulated to account for boundary conditions and to maintain calibration. Although the study area of the model corresponds to the area included in the ground-water flow model, the focus of this investigation was Union County.
An annual increase of 1.03 percent in water use was used for the baseline projection simulations for the Coffee Sand and Eutaw-McShan aquifers for years 2001 to 2050. In the New Albany area, simulated drawdowns in the Coffee Sand aquifer were about 65 feet below year 2000 water levels. At the center of a cone of depression in the New Albany area, simulated drawdowns in the Eutaw-McShan aquifer were about 120 feet for the year 2050. The resulting projected water level at the center of the drawdown cone in the New Albany area is between 500 and 550 feet above the top of the Eutaw-McShan aquifer.
The normal- and high-growth projection simulations for the Coffee Sand and Eutaw-McShan aquifers used the normal- and high-growth output data from the water-demand model for Union County, and an annual increase of 1.03 percent in water use for years 2001 to 2050 for other areas in the model. For normal-growth projections, simulated drawdowns in the Coffee Sand aquifer in the New Albany area were about 65 feet below year 2000 water levels. For high-growth projections, simulated drawdowns in the Coffee Sand aquifer were about 75 feet below year 2000 water levels.
For the Eutaw-McShan aquifer, normal-growth projections resulted in a cone of drawdown centered on the New Albany area of Union County. The cone shows a maximum drawdown at the center of about 135 feet. The resulting projected water level for the year 2050 at the center of the drawdown cone in the New Albany area is between 500 and 550 feet above the top of the Eutaw-McShan aquifer. For high-growth projections, the cone shows a maximum drawdown at the center of about 190 feet. The projected water level for the year 2050 at the center of the drawdown cone in the New Albany area is between 450 and 500 feet above the top of the Eutaw-McShan aquifer.