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Corral Hollow Landfill 2 San Joaquin County Third Quarter 1. MW-4 Statistical Analysis: The County reports a decreasing trend of pH in MW-4 and that, "Significant trends in pH can indicate landfill impact. However, since the mean value of pH in downgradient MW-4 is greater than the mean value of pH in upgradient SB-1,the pH trend at MW-4 is inconclusive." Intrawell analysis is currently being performed for monitoring wells. Groundwater chemistry between MW-4 and SB-1 are very different. The report also states an increasing trend of nitrate was found in MW-4 and proposes the impact could be from cattle that graze the adjacent property along the Corral Hollow Creek(a.k.a. South Paddy Creek?-see Figure 2). The report also states that surface water samples haveshown high amounts of TDS and sulfate, similar to that found in MW-4; therefore, the County suspects that the nitrate trend at MW-4 is caused by leaching of cow waste into the groundwater. The County proposes to continue to monitor nitrate concentrations in samples from MW-4 and from surface water to assess the situation. We do not concur with the County's assessment and provide the following observations of the data: ■ According to Figure 2, there is a Retention Pond between the Corral Hollow Creek and MW-4. This could act as a hydraulic barrier to groundwater recharge from the creek during precipitation events. ■ The intermittent creek only flows during storm events when constituents are most likely diluted. Recharge water laden with nitrate would be depicted as a slug of nitrate during winter months. If nitrate from animal waste was the source, the data would not depict an increasing trend,but a flat trend caused by the constant source. This is not evident in the data. Furthermore, WDRs state that, "The facility receives an average of 8 inches of precipitation per year as measured at the Tracy 2 SSE weather station." There is so little rainfall that it is difficult to maintain vegetation on the cap of the landfill. ■ MW-4 is 150 feet from the landfill in comparison to the creek which is>200 feet from the well. The groundwater gradient is from the landfill towards MW-4. Corral Hollow Creek is topographically downgradient from MW-4. The County may choose to prepare a cross- section showing the relationship of the stream bottom to MW-4. ■ The attached time-series graphs and Summary Statistics show water quality is different between surface water and groundwater at MW-4. SW-2 -4 Constituent Statistical Mean Statistical Mean Difference Nitrate as Nitrogen 0.5332 19.6 19.23 Total dissolved solids 1077 1747 654 Sulfate 362.1 642.4 304.1 i