TECHNIQUES USED TO DETERMINE WATER SOURCES TO THE INDEPENDENCE, MISSOURI WELL FIELD

The city of Independence, Missouri, operates a water-supply well field in the Missouri River alluvial aquifer. Ground-water flow simulation, particle tracking, and chemical and isotopic data in a two-component chemical mixing equation were used to determine sources of water to the well field.

Steady-state flow budget analysis indicates 86.7 percent of well-field pumpage is induced inflow (from the Missouri River), 6.7 percent is recharge (from precipitation to ground water) and 6.6 percent is estimate uncertainty caused by model cells too large to uniquely define ground-water flow divides. Particle-tracking results indicate induced inflow is 82 percent, recharge is 10 percent, and estimation uncertainty is 8 percent. Particle tracking was used to determine the percent of source water as a function of travel time from the source. Source water that traveled 1 year or less is 8.8 percent, 2 years or less is 10.3 percent, 5 years or less is 36.5 percent, 10 years or less is 42.7 percent, and 25 years or less is 71.9 percent. Mixing equation results indicate induced inflow in source water ranges from 49 to 80 percent, and equation sensitivity ranges from 0 to plus or minus 35 percent. The average of all mixing equation results indicate induced inflow is 61 percent of source water.

Estimates from ground-water simulation techniques are larger and errors are smaller than those from the mixing equations. Aquifer hydrology is complex near the well field and particle-tracking estimates probably are more reliable than the other ground-water simulation methods or the mixing equation results. More reliable mixing equation results can be obtained by increasing the number of samples and collecting samples during a variety of flow conditions. In the absence of a calibrated ground-water flow simulation, the mixing equation can provide a reasonable estimate of well-field water sources at relatively low cost.