Accurate estimates of the nitrate concentrations in ground water in west Salt River Valley are needed to better manage ground water affected by nitrate. Statistical analyses were done to establish the best statistical method to produce these estimates. Three sets of ground-water data for different time periods --1975-77, 1980-85, and 1986-90--were used to analyze spatial and temporal variations in concentrations of nitrate in ground water.
The use of inverse-distance squared weighting, radial-basis function, kriging, and cokriging were evaluated for estimating nitrate concentrations in ground water. From an analysis of the cross-validation results, cokriging maps resulted in the best estimates, and they were accepted as being the most reliable. Cross-validation results also indicated that nitrate cokriged best with magnesium for 1975-77 and 1986-90 and with calcium for 1980-85. Kriging results consistently were almost as reliable as any of the cokriging results. Because of the difficulties inherent in the cokriging process, kriging, although not optimal, was the fastest way to obtain reasonably good results.
In 1980-85, cokriged nitrate concentrations exceeded 20 milligrams per liter in a 12-square-kilometer area in Phoenix and Glendale and exceeded 10 milligrams per liter in a 280-square-kilometer area that extended to the Salt River. In 1986-90, nitrate concentrations along the entire reach of the Salt River in west Salt River Valley were less than 10 milligrams per liter and were smaller probably as a result of recharge from the Salt and Gila Rivers in 1982. Farther north in Phoenix and Glendale, the area in which nitrate concentrations exceeded 10 milligrams per liter expanded to 490 square kilometers for 1986-90. In Buckeye Valley, nitrate concentrations exceeded 10 milligrams per liter in an area of 300 square milometers for 1980-85 from the Gila River in the early 1980's but possibly could be an artifact of the different data distributions associated with each data set. In the Phoenix area, cokriged nitrate concentrations for 1975-77 exceeded 10 milligrams per liter in a 290-square-kilometer area and exceeded 20 milligrams per liter in a 1.4-square- kilometer area.
Additional publication details
USGS Numbered Series
Statistical analysis of nitrate in ground water, West Salt River Valley, Arizona
Water-Resources Investigations Report
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