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Sources of Nitrate Septic systems and dairies are significant sources of nitrate found in ground water. Farming activities using soil fertilization where sandy soils are present may also contribute to nitrate contamination, especially in shallow unconfined aquifers. The proposed project will add nitrate to the groundwater through use of an on-site septic disposal system. Although there are no data to determine groundwater gradients at the site, it is likely to follow the slope of the ground,generally from south to north. The distance between the property and a groundwater boundary(coastal hills) is short,so areas/activities that may contribute a nitrate load upgradient are limited. According to satellite photography, agrarian use is minor,and there are only 5 to 10 houses on large AR zone lots that are assumed to have on-site septic systems. The project consists of commercial warehousing,so the septic load will include bathrooms (toilets,sinks, urinals), and small breakroom area (sinks,dishwasher). Since the sewage characteristics are not the same as residential (which would include wastewater from showers,clothes washing,additIQ��al food preparation, hygiene activities),the nitrate concentration is greater. Thus,a value o 70 mg/II as N was used for the study. ? I L✓ Contribution of Rainfall An estimate of the quantity and quality of rainfall was required for the nitrate loading analysis. The Hantzsche and Finnemore method utilizes the"average recharge rate of rainfall' in calculating potential adverse nitrate impacts to the aquifer. It is assumed that not all rainfall will percolate to the aquifer, as some water may be lost to evaporation or transpiration. However, most of the rainfall in California is during the cooler months when evaporation is less. Hence, if water is held in detention for long periods,and the holding basin soils are permeable, much of the water will percolate. Rainfall recharge (or deep percolation of rain) is a significant factor in the nitrate loading estimate in the Hantzsche and Finnemore model. There are a variety of rainfall data that span various timeframes for the Tracy area. A revie of the rainfall_data from various sources indicate a range of average rainfall somewhere betwee 11 in es d 14 i es per year. For purposes of this report,the more conservative valu 1 ches was used for determining nitrate loading. Due to requirements for new developments to retain the site runoff, nearly all of the storm water for the parcel will be held in on-site detention basins. Additional runoff from the adjacent property flows into the project's detention basins, resulting in a total watershed area of 58.03 acres. (See Exhibit B). As such, it is estimated t a °o to 80%oof the rainfall will percolate into the ground. Historically, most of the precipitation that fell on the property would run off,since the site slopes at approximately 1%to 2%, and the top soil has a high clay content. Thus, development will greatly increase the volume of groundwater rechjarge at the site. For purposes of this study,the more conservative valu 46 0%w s u d determining nitrate. loading. The nitration concentration for rainfall,wa4 estimat e 1 mg/L- (Hantzsche and Finnemore). LBA Nitrate Loading Study/Jan 2017 pg.4