Ground Water Monitoring Activities Arkansas River Valley Alluvial Aquifer 94-95

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0293 � 1 1995 CoNmmmo r SAwi nvG In August 1995, a confirmation sampling program was conducted to confirm pesticide detections and elevated nitrate levels measured in 1994. Analysis of the nitrate data had indicated nineteen (19) wells in three areas where nitrate levels exceeded the drinking water standard of 10.0 mg/L. The pesticide data revealed twelve (12) wells with Atrazine, one well with Metolachlor and Atrazine, and another with 2,4 -D. The confirmation sampling program consisted of resampling thirty two (32) wells that had shown either a pesticide detection or nitrate level above the standard in the 1994 sampling. The resampling program was designed to determine if the contamination originally detected was representive of the groundwater quality at that site or only a conincidence of timing of the sampling. The only change in field or laboratory procedures from 1994 to 1995 was a decrease in the method detection level for Atrazine from 0.5 to 0.1 micrograms per liter or parts per billion. The lowest level of Atrazine which the laboratory could quantify was thus more sensitive by a factor of five. As a result, those detections reported as traces (atrazine present in the sample, but to low to quantify) in 1994 are reported in micrograms per liter in the 1995 data. Twenty eight (28) wells were sampled in 1995. Two wells sampled in 1994 were out of service when revisited in 1995, and the sampling of two others could not be scheduled within the allowable time frame. Table 3 summarizes the 1995 results and compares them to the previous year. The 1995 results confirmed the quality of the 1994 field and laboratory work. Of the nineteen (19) wells that had nitrate levels in excess of the EPA drinking water standard of 10 mg/L in 1994, four decreased below the standard. Two wells resampled for a pesticide detection increased above the standard. The high mobility of nitrate combined with this hydraulically active ground water system can produce significant swings in nitrate concentrations from year to year. But overall, the differences between the 94 and 95 values are statistically insignificant. The resampling indicates little or no change in nitrate levels from one year to the next in those wells that were sampled both years. The data does demonstrate the fluctuations one can normally expect in ground water nitrate concentrations, and confirms that if proposed action is tied to a numerical standard, long term monitoring is recommended. The pesticide resampling showed four wells with a trace amount of Atrazine (up to but not over 0.49 ug/L) in 1994, falling below the 1995 detection limit of 0.1 ug/L. One well resampled for nitrate picked up a hit of Atrazine. Another well increased from a trace level to 4.20 ug/L which is over the standard of 3.0. This is the only occurrence of pesticides at or above a water quality standard in the Arkansas Valley survey. 10