New Mexico Geological Society Annual Spring Meeting — Abstracts

Managed aquifer recharge is being implemented at sites across New Mexico as an important water resource management tool, to recharge and store surplus water within aquifers, and to recover the water for later use. During the site selection process, potential geochemical reactions that may affect water quality are an important consideration. Several examples from managed recharge sites in New Mexico are presented that illustrate how water quality may be influenced during the aquifer recharge process.

New Mexico has several managed aquifer recharge project sites that are in varying stages of project development, ranging between characterization of hydrogeology and geochemistry, to permitting and operations. Water managers enhance aquifer recharge using surface and subsurface approaches such as:

Surface methods: infiltration galleries, ephemeral stream reaches, and basins

Subsurface techniques: vadose zone wells and ASR wells

Sources of water include bank filtered or treated surface water, reclaimed water, or groundwater. Benefits of enhanced aquifer recharge include maximizing storage capacity, improving management of seasonal surplus water supply, reducing evaporation, avoiding land subsidence, minimizing drawdown and developing drought reserves.

During the aquifer recharge process, water will either infiltrate through the vadose zone to reach the water table or be injected directly into the aquifer. Each method allows for various geochemical reactions to occur between the injected and receiving waters, or injected water and aquifer sediments. Water moving through the vadose zone may react with pore water and minerals in the vadose zone sediments. During injection, the recharge water pushes groundwater away from the well, and this is often conceptualized as a bubble of injected water forming around the wells.

Geochemical compatibility of the injected water with both the receiving water and the aquifer material can be predicted using geochemical models and calculations for binary mixing and saturation indices (SI). A range of potential outcomes can be identified including probable water quality at the time of recovery, treatment requirements, and operational difficulties that should be planned for or potentially avoided through treatment or engineering solutions.Water quality may be improved, maintained, or degraded. The mixing of different water chemistries and aquifer matrix reactions may affect the water quality of the recovered water. Geochemical methods may also be used to trace injected water including using stable isotopes, ion ratios, sulfur hexafluoride (SF₆), and disinfection byproducts. Examples include:

Mixing of water in the Tesuque Formation aquifer: elevated fluoride, arsenic and uranium in groundwater mixing with treated surface water may require treatment upon retrieval

Iron and manganese in the Santa Rosa Sandstone-Chinle Formation aquifer system: potential mobilization of iron and manganese needs to be predicted for the final water quality

Arsenic in the Santa Fe Group aquifer system: reactions between sediments and injected water may dissolve iron coatings on sediments releasing iron and adsorbed arsenic

Tracing injected water in the Santa Fe Group aquifer system: SF₆ was injected into the injection water as a conservative tracer to monitor movement in the aquifer