Methods for Identification of Potential Future Threats to Louisiana’s Groundwater Supplies

Pierre Sargent, U.S. Geological Survey, Lower Mississippi-Gulf Water Science Center, Baton Rouge Office, Suite 120, 3535 S. Sherwood Forest Blvd., Baton Rouge, LA, 70816: psargent@usgs.gov and Angela L. Collier, U.S. Geological Survey, Lower Mississippi-Gulf Water Science Center, Baton Rouge Office, Suite 120, 3535 S. Sherwood Forest Blvd., Baton Rouge, LA 70816: acollier@usgs.gov

Identification of potential threats, such as declining water levels or saltwater encroachment, to Louisiana’s groundwater supplies is critical for the sustainability and management of these resources. During the period from 1996-2005, water levels in the Sparta aquifer, located in north-central Louisiana and southern Arkansas declined at rates approximating 1 foot/year and concerns arose about the long-term viability of the aquifer for municipal supplies and other uses. Because of these concerns, Union County, Arkansas, initiated a conservation plan aimed at reducing withdrawals from the Sparta aquifer. Industry closures in north-central Louisiana combined with the implementation of the conservation plan in Arkansas have reduced stress on the aquifer. Water-level recovery in the Sparta aquifer demonstrates that groundwater supplies can be replenished with proper management. In this presentation, three methods for early identification of potential threats to groundwater supplies in Louisiana are discussed. They include the use of the U.S. Geological Survey’s Groundwater Watch website, a water-balance approach, and a decision- matrix approach.

As groundwater levels in Louisiana respond to changes in groundwater withdrawal rates, water-level data from Groundwater Watch can be a useful tool in identifying potential aquifer overuse. However, discontinuities and lateral and vertical anisotropy in an aquifer can make it difficult to infer aquifer stress from declining groundwater-level trends. A water-balance approach, with a comparison of groundwater withdrawals against estimated potential yields in an aquifer, offers another means to identify potential threats to groundwater supplies. A 2003 Illinois State Water Survey report used the ratio of groundwater withdrawals to groundwater yield to help identify areas of potential need for water-resources management. However, the lack of sustainable groundwater recharge information and the complexity of aquifer systems in Louisiana make this approach difficult to implement. The last approach uses a decision matrix to identify parishes with potential threats to groundwater supplies. The method qualitatively ranks parishes at risk based on selected factors related to water availability and potential water demand. Parishes are given points for the following water-availability factors: (1) parishes with multiple water sources, (2) parishes utilizing multiple aquifers, and (3) parishes that are in proximity to the Red or Mississippi Rivers. Points are assigned to parishes for potential water demand factors based on: (1) population density, (2) the presence of a power generation plant, and (3) industrial withdrawal rates with the greater the number of points the lower the risk.

Key words: groundwater, water level, water use, Louisiana

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