San Xavier district Recharge Water Quality Study
This study will evaluate the quality changes of the recharge water as it percolates through the vadose zone material. The changes could be both beneficial and detrimental to the original quality. Chemical equilibrium models will be used to evaluate potential chemical interactions between the recharge water quality and the aquifer water quality.
Need and Benefit
This study will be coordinated with the San Xavier District, Tohono O'odham Nation, Recharge Feasibility Pilot Investigations that will use Central Arizona Project (CAP) water to recharge existing aquifers in the district. The proposed work will evaluate the chemical interactions of the CAP water within the vadose zone and the local aquifer water. The CAP water contains some natural organic matter (NOM), which is a general reference for total organic carbon (TOC) and dissolved organic carbon (DOC) and provides an opportunity to study its fate in the recharge-storage-recovery aquifer environment. If the NOM builds up in the aquifer and the recovered water is used for domestic purposes, harmful byproducts could be formed from using chlorine as the disinfectant chemical.
The planned use of the recovered water is for irrigation purposes. However, it may be used to supplement potable water supplies-where the fate of NOM material is important. It is important to understand the fate of NOM during recharge and subsequent recovery. Therefore, this study will benefit other recharge projects that use recharge water with NOM when the planned recovery use is for potable water supplies.
Existing water quality data and additional water samples will be collected by the area office or District personnel for analysis for the listed parameters to meet the research needs.
The District and Reclamation have established ground water monitoring piezometers at the potential recharge sites. The ground water samples will be analyzed for the listed water quality parameters. Any additional water quality parameters needed for the study will be identified and determined from the samples collected. The additional costs of analysis will be covered by this study. The proposed parameters for this study are listed as follows: pH, sulfate, bicarbonate, chloride, nitrate, calcium, sodium, manganese, total dissolved solids, total organic carbon, dissolved organic carbon, free chlorine residue, Trihalomethanes (THM), and haloacetic acids (HAA). If the mixed CAP and aquifer water that has more than one milligram per liter (mg/L) TOC, it will be bench tested to determine disinfection byproduct production.
Pre-recharge aquifer water samples will be collected and analyzed for the above list of parameters. The chemical makeup of the CAP water will be used to project possible chemical interactions between the recharge water and the aquifer water. This data will be used in a chemical equilibrium model to predict potential chemical interactions in the aquifer. Once the recharge water reaches the aquifer, additional aquifer samples will be collected and analyzed for comparison with the chemical model results.
The bore hole core logs and descriptions of the core material will be obtained from the regional office by D-8570. The soil chemical analysis can be conducted by the Technical Service Center (TSC) soils laboratory along with leaching tests. The core material and log will be reviewed and vadose zone material selected for leachate testing. The core chemical testing will cover the above listed parameters and a series of trace elements. The trace elements are selenium, arsenic, boron, copper, lead, mercury, chromium, cadmium, silver, and zinc. It is anticipated that a sample will be analyzed for each significant core material change.
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