Harte Research Institute: A Statement on Surface Discharge of Brine Water from Brackish Groundwater Desalination in the Coastal Zone

A Statement on Surface Discharge of Brine Water from Brackish Groundwater Desalination in the Coastal Zone

Introduction

Desalination of brackish groundwater to produce freshwater is an increasingly used option for cities and counties on the Texas coast that are facing water availability shortages. In this process, brackish water with a total dissolved solid (TDS; mostly as salt) content of roughly 1,000-10,000 mg/L (seawater TDS is ≥30,000-35,000 mg/L) is drawn to the surface from aquifers that are typically 500-1500 feet deep. The water is then processed to remove salts, usually by reverse osmosis membrane filtration, resulting in freshwater that enters the water supply, as well as a “waste” product, brine water, which has a much higher TDS/salinity (=salt content) than the original brackish groundwater. Many cities and counties in Texas have disposed of the waste product by deep-well injection; however, surface discharge into adjacent streams and rivers is now being proposed as an option as well. Yet, there have been virtually no studies on the potential environmental effects of surface discharge of brine water to streams and estuaries.

Potential Environmental Costs and Benefits of Surface Discharge of Brine Water

The relative environmental costs versus benefits of surface discharge of brine water from brackish groundwater desalination depend on several factors, including:

1. The TDS of the brackish groundwater;
2. The TDS and volume of the brine water being discharged, including cumulative effects from multiple discharge sources;
3. The water quality of the brine water, including metal content, nutrient (nitrogen, phosphorus) content, dissolved oxygen levels, water temperatures, etc.; and
4. The TDS content and volume of the receiving water body, which is often very variable and determined by:
   1. Location on the Texas coast: TDS in coastal streams and estuaries is much lower on the central-north coast (from roughly San Antonio Bay to Sabine Lake) compared to the south coast (Lower Laguna Madre to Corpus Christi Bay).
   2. Hydrologic conditions, i.e., drought, average, wet conditions.
   3. Seasonality, especially in relation to predominant precipitation patterns.

Environmental effects from the surface discharge of brine water may be negative, neutral, or even positive depending upon the variable. For example, adverse effects on aquatic life would be expected from the discharge of brine water with poor water quality, such as high metal or nutrient content, low dissolved oxygen, or high water temperature. Negative effects of brine water discharge on surface waters are also expected when the TDS content of the brine water is significantly higher than the ambient levels of the receiving water body, potentially leading to habitat degradation and mortality of aquatic life. Many organisms have a narrow window for TDS/salinity tolerance, and the addition of concentrated brine to an aquatic ecosystem may lead to exceeding this survivability window. However, in cases where the brine water has a lower TDS content than conditions in the receiving waterbody, there may be benefits such as dilution of water with high ambient TDS and/or water with poor water quality, as well as increased flushing of otherwise stagnant water. Even in this scenario, changes to aquatic habitat and biological communities can be expected and should be carefully considered with input from stakeholders, resource managers, and scientific experts.

HRI Recommendations for Siting of Brackish Groundwater Discharge Points to Surface Waterbodies

Much of the potential effects of brine discharge, positive, neutral, or negative, largely depend on the content of the brine discharge as well as the receiving waterbody. Hence, any siting decision must begin by characterizing the TDS and water quality in the brackish groundwater and subsequent brine, as well as determining these factors along with the habitat and biological community of the receiving surface waterbody. Effects also will be dictated by the volume of brine discharge relative to the receiving waterbody.

With these factors in mind, it is highly recommended that additional characterization steps be taken prior to project implementation:

1) hydrologic modeling to quantify the effects of brine discharge on the salinity and circulation/mixing of the receiving waterbody under various hydrologic, precipitation, and seasonal timing scenarios; and

2) ecological modeling to understand effects on habitat, aquatic life, and essential aspects of the ecosystem. This would give resource managers and stakeholders reasonable confidence that negative impacts are unlikely and also may lead to outcomes on alternative siting options with environmental benefits. To the extent that negative impacts are found, this would be an initial step in designing a mitigation strategy.

3) Subsequent characterization of the biological communities that may potentially be affected near the model-projected impact outfall (and its full plume) as well as control areas.  This biological sampling should be done both before and after the discharge occurs.