SR0033856_SSNL - Laserfiche WebLink

Home Browse Search

r . LUSIONS AND RECOMMENDATIONS FOR IMPACT MITIGATION an Cobe hypothesized that the observed nitrate concentrations within "Valpico Section" are coming into equilibrium. Sources contributing nitrate to the underlying groundwater include indigenous soil concentrations from decomposing organic matter, rainfall, upgradient agricultural fertilizer inputs, septic systems (particularly from sumps and pits), and lawn/landscape fertilization. Factors decreasing the groundwater nitrate concentration include denitrification, groundwater movement (both vertical and horizontal), well pumping and well hydraulics, and cleaner water recharge contributing to a dilution effect. Therefore, if each of these sources and attenuating factors could be quantified on a mass balance basis, it may be that nitrate input is now roughly equivalent to output, or attenuation. Given that agricultural irrigation recharge is the largest single contributor to groundwater nitrate concentrations based on my own observations, and since the surrounding land has been farmed for several decades, it is assumed that the nitrate concentrations in the underlying groundwater should be much higher than those observed, if the attenuating factors were not significant. This is what was observed with the soil chemical analyses. The denitrification potential is significant due to the comparatively high clay content(24%) of the indigenous soils, the higher soil pH, high soil moisture content and organic fraction content. On a mass-to-mass ratio basis,there can be a dilution effect of nitrate concentration from the resultant recharge water (Nr) as long as the underlying aquifer(s) have a higher nitrate concentration than the recharge water. NITRATE-NITROGEN IMPACT MITIGATION Given that the indigenous soils under the subject property are conducive for denitrification, the new septic system for this Parcel should be installed to take advantage of these favorable soil characteristics. It is proposed to install two separate leachfields for each of the two residential structures. Leachline trenches should be installed as high in elevation as possible to take advantage of evaporation and evapotranspiration. The plumber for the project must be made aware of the house elevations so that the elevations between the influent from the two houses can be correlated with the most distant point of the leachlines. Deep rooted grasses may be planted over the leachfield. Deciduous and evergreen plants should be planted no closer than 15 feet from any leachline to prevent serious root infiltration into the leachlines. The scientific literature suggests that denitrification may be enhanced by placing proposed leachlines higher in elevation toward the soil surface, particularly since the surface soils contain approximately 24% of the clay soil fraction. Additionally, during the rainy season, November through March,nitrification is suppressed during these colder months of the year, so it would be beneficial for the leachlines to be placed high in elevation where the soil/effluent interface is colder. These procedures described above should theoretically provide a triple nitrate mitigation system of 1.) denitrification/clay absorption, 2.) plant root system uptake, and 3.) evaporation/evapotranspiration. Page -7- CheRwy Con suiting