Ground-water monitoring plan, water quality, and variability of agricultural chemicals in the Missouri River alluvial aquifer near the City of Independence, Missouri, well field, 1998-2000

A detailed ground-water sampling plan was developed and executed for 64 monitoring wells in the city of Independence well field to characterize ground-water quality in the 10-year zone of contribution. Samples were collected from monitoring wells, combined Independence well field pumpage, and the Missouri River at St. Joseph, Missouri, from 1998 through 2000.

In 328 ground-water samples from the 64 monitoring wells and combined well field pumpage samples, specific conductance values ranged from 511 to 1,690 microsiemens per centimeter at 25 degrees Celsius, pH values ranged from 6.4 to 7.7, water temperature ranged from 11.3 to 23.6 degrees Celsius, and dissolved oxygen concentrations ranged from 0 to 3.3 milligrams per liter. In 12 samples from the combined well field pumpage samples, specific conductance values ranged from 558 to 856 microsiemens per centimeter at 25 degrees Celsius, pH values ranged from 6.9 to 7.7, water temperature ranged from 5.8 to 22.9 degrees Celsius, and dissolved oxygen concentrations ranged from 0 to 2.4 milligrams per liter. In 45 Missouri River samples, specific conductance values ranged from 531 to 830 microsiemens per centimeter at 25 degrees Celsius, pH ranged from 7.2 to 8.7, water temperature ranged from 0 to 30 degrees Celsius, and dissolved oxygen concentrations ranged from 5.0 to 17.6 milligrams per liter.

The secondary maximum contaminant level for sulfate in drinking water was exceeded once in samples from two monitoring wells, the maximum contaminant level (MCL) for antimony was exceeded once in a sample from one monitoring well, and the MCL for barium was exceeded once in a sample from one monitoring well. The MCL for iron was exceeded in samples from all monitoring wells except two. The MCL for manganese was exceeded in all samples from monitoring wells and combined well field pumpage.

Enzyme linked immunoassay methods indicate total benzene, toluene, ethyl benzene, and xylene (BTEX) was detected in samples from five wells. The highest total BTEX concentration was less than the MCL of toluene, ethyl benzene, or xylene but greater than the MCL for benzene. Total BTEX was not detected in samples from any well more than once. Atrazine was detected in samples from nine wells, and exceeded the MCL once in a sample from one well. Alachlor was detected in samples from 22 wells but the MCL was never exceeded in any sample.

Samples from five wells analyzed for a large number of organic compounds indicate concentrations of volatile organic compounds did not exceed the MCL for drinking water. No semi-volatile organic compounds were detected; dieldrin was detected in one well sample, and no other pesticides, herbicides, polychlorinated biphenyls, or polychlorinated napthalenes were detected.

Dissolved ammonia, dissolved nitrite plus nitrate, dissolved orthophosphorus, alachlor, and atrazine analyses were used to determine the spatial and temporal variability of agricultural chemicals in ground water. Detection frequencies for dissolved ammonia increased with well depth, decreased with depth for dissolved nitrite plus nitrate, and remained relatively constant with depth for dissolved orthophosphorus. Maximum concentrations of dissolved ammonia, dissolved nitrite plus nitrate, and dissolved orthophosphorus were largest in the shallowest wells and decreased with depth, which may indicate the land surface as the source. However, median concentrations increased with depth for dissolved ammonia, were less than the detection limit for dissolved nitrite plus nitrate, and decreased with depth for dissolved orthophosphorus. This pattern does not indicate a well-defined single source for these constituents. Dissolved orthophosphorus median concentrations were similar, but decreased slightly with depth, and may indicate the land surface as the source. Seasonal variability of dissolved ammonia, dissolved nitrite plus nitrate, and dissolved orthophosphorus concentrations is not well-defined for samples grouped by month. Individual sample results from the Missouri River and the combined Independence well field pumpage show a seasonal trend that may indicate the effect of induced recharge from the Missouri River on the well field. Individual results and samples from seven wells indicate relatively constant dissolved ammonia concentrations, low dissolved nitrite plus nitrate concentrations in all but one well, and a more well-defined seasonal variation of dissolved orthophosphorus than either dissolved ammonia or dissolved nitrite plus nitrate.

Larger detection frequencies in the shallow depth intervals and the large maximum concentrations of alachlor and atrazine in the shallower wells indicate that the source of these chemicals to shallow ground water most likely is the land surface rather than induced river recharge. Seasonal variability of alachlor is not well-defined from samples grouped by month. Individual results from the combined Independence well field pumpage and samples from seven wells indicate variation in both alachlor and atrazine with time, but the degree and timing of the variation is not consistent. This inconsistency most likely is caused by varying rates of transport from the land surface to wells, different rates of degradation of both alachlor and atrazine in soil and ground water, the timing and area of application, and multiple source areas in and around the Independence well field. Alachlor and atrazine concentrations in the Missouri River at Hermann, Missouri, increased in spring, greater concentrations continued until late summer or early fall, and lower concentrations occurred in late fall and winter. Alachlor and atrazine concentrations most likely followed a similar trend in the Missouri River near the Independence well field. The lack of a seasonal trend similar to that of the Missouri River in the combined Independence well field pumpage and the monitoring wells may indicate that the source of alachlor and atrazine to ground water is the land surface rather than the Missouri River.