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  California Water Science Center

Warren Valley Nitrate Study

Problem: In an attempt to alleviate ground- water overdraft, the Hi- Desert Water District (HDWD) instituted an artificial recharge program in 1995. The artificial recharge resulted in water levels recovering about 150 feet between 1995-97. However, accompanying the recovered water levels there has been an increase in nitrate concentrations in some production and monitoring wells. For example, nitrate-n increased from 2 mg/L to 30 mg/L in a shallow monitoring well; the EPA MCL is 10 mg/L for drinking water. There are two potential sources of nitrate; septic tank leachate, or mineralization of organic material and the subsequent leaching of the natural soil nitrate. Therefore, the source of nitrate and the chemical, biological, and hydraulic processes that affect nitrate concentration need to be determined. In addition, the amount of nitrate reaching the water table, the degree of mixing, and the vertical distribution of nitrate in the saturated zone must be determined.

Objectives: 
(1) To determine the source of nitrate in the Warren Valley ground-water basin. 
(2) To determine the fate and transport of the nitrate in the ground-water basin.
(3) To develop simulation-optimization modeling techniques to evaluate various management options that "best" meet the needs of HDWD.

Relevance and Benefits: This project will help meet the USGS goals of (1) advancing the knowledge of the regional hydrologic system and (2) providing water-resources information that will be used by multiple parties for planning and operational purposes. Regarding goal 1, the data collection and model development approaches have aided in identifying the source of nitrates and have showed the importance of faults to the hydrologic regime. Regarding goal 2, the simulation-optimization model will provide decision makers a method to evaluate alternatives tground-water management scenaios.

Approach: There are three main elements to the study approach; (1) data collection, (2) development of a ground-water flow and solute-transport model, and (3) development of a simulation-optimization model.

Data Collection:  Water-level and water-quality data will be collected from selected production wells in the basin and monitoring wells near the infiltration ponds. Most of the samples will be analyzed for major ions, nitrogen species, and organic carbon. Selected samples will be analyzed for isotopes of nitrogen, hydrogen, and oxygen; caffeine; UV absorbence; and fluorescence in order to distinguish the sources of water. Samples will also be collected from potential nitrate sources including septic-tank water, recharge water, and sediment.

Ground-water Flow and Solute-Transport Model:  A ground-water flow and solute-transport model will help to better understand the dynamics of the Warren Valley ground-water basin and will allow decision-makers to investigate the potential impacts of various pumping/recharge strategies. A conceptual model will first be developed. A numerical model will then be developed based on the conceptual model. The unknown flow and transport parameters will be calibrated by attempting to match simulated water levels and nitrate concentrations with observed data using observed stresses.

Simulation-Optimization Model:  A simulation-optimization modeling approach will be used to identify the "best" strategy, or strategies, to manage HDWD's ground-water resources. The USGS-developed flow and transport model will be used to represent the physics of the system. In close cooperation with HDWD personnel, the objective function and constraint set will be identified and mathematically formulated. The optimization problem will be solved and the optimal solution(s), if it exists, will be identified. A sensitivity analysis will identify a suite of management strategies.

Progress and Significant Results in FY 1999: Analysis of water-quality data indicates that the source of nitrate in the Warren Basin ground-water basin is the septic systems. The ground-water flow model has been completed, and the solute-transport model is currently being developed.

Plans for FY 2000: Complete the solute-transport model and incorporate the flow and transport models into an optimization model.

Number: CA534
Location:
Cooperating Agencies: Mojave Water Agency
Project Chief: Tracy Nishikawa
Period of Project: October 1997 through September 2000
Team:

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Page Last Modified: Thursday, 05-Jan-2012 15:16:37 EST