Abstract
Nutrient (Nitrogen and Phosphorus) chemistry downgradient from on-site wastewater treatment system(s) (OWTSs) was evaluated by conducting a groundwater study surrounding the largest of the three Santa Clara River Lakes, Lake Elizabeth. Lake Elizabeth is listed on the “Impaired Water List” for excess nutrients and is in close proximity to more than 600 OWTS. The primary objective of this study is to determine if OWTSs discharge into shallow groundwaters are responsible for excess nutrients transported and discharged into Lake Elizabeth. Electrical Resistivity Tomography (ERT) surveys were conducted to determine monitoring well locations, depth to bedrock, and water level depths. Seven monitoring wells were constructed by the United States Geological Survey (USGS) drilling research project using an auger drill rig along with three hand auger drive points. These ten monitoring wells in addition to Lake Elizabeth were sampled in February, June, July, and September of 2020. Additionally, two public supply tanks were sampled as a fingerprint for imported water.ERT transects and borehole geophysical measurements show that there are both low and high resistivity materials in the subsurface. Resistivity was low (0-40 ohm-meter) at shallow depths (0-30 ft below land surface (bls)), highly resistive (up to 100 ohm-meter) at mid depths (30-50 ft bls), and moderately resistive (40-70 ohm-meter) at deeper to total well depths (30-90 ft bls). The majority of aquifer material was characterized from drill cuttings as sandy silt, with mixed clays and minor gravels. These aquifer materials were estimated to have a median hydraulic conductivity of 3.28 x 10-3 – 16.4 ft/d, according to Heath (1983) hydraulic properties according to soil type. Groundwater levels suggest primary groundwater recharge, aside from anthropogenic sources such as OWTS discharge, to be from precipitation and storm runoff.
The collection of water samples included major ion chemistry, nutrients, dissolved organic carbon (DOC), isotopes, and age tracers. A principal component analysis model was created by the USGS demonstrates that DOM optical properties are tracking different sources of DOM in septic influences wells vs Lake Elizabeth. Recharge from OWTS and irrigation from imported water was estimated, using a simple two-part mixture equation using imported water and estimated native groundwater δD values, imported water was calculated to account for an average of 34% of the groundwater. Monitoring wells, that exhibit a strong imported water signal influence contain estimated percentages of approximately 49-71% of imported water in groundwater. The age of groundwater was determined to generally be young modern waters, recharging since 1952 at a median temperature of 13 (oC). Nitrate (NO3-) concentrations in groundwater samples ranged from less than the detection limit (0.01 mg/L as N), to approximately 24 mg/L as N. Elevated concentrations of ammonium in Lake Elizabeth are assumed to be due to avian waste products and are significantly higher than groundwater concentrations. NO3- was not detected in 3 of the 12 sites sampled during the study, two wells and Lake Elizabeth. Dissolved organic carbon concentrations are suspected to influence redox conditions of groundwater and suggest nitrogen removal is likely occurring in the majority of the wells and, thus, down gradient. Consequently, it is possible that majority of the removal of NO3- will occur before groundwater reaches the south side of the lake. A majority of the wells contained δ15N-NO3 and δ18O-NO3 values consistent with denitrification and septic source. However, a monitoring well on the north side of Lake Elizabeth contains high concentrations of NO3- and no evidence of denitrification, it is unclear if the high NO3- concentrations are septic or soil derived. Consequently, this NO3- may be delivered to the lake through groundwater discharge if NO3- is not removed from the system by denitrifying bacteria by the time the groundwater reaches the surface water which cause the water to become more alkaline, thus limiting the ability for orthophosphate and ammonium (NH4+) ion sorption onto oxide and clay minerals. Thus, NH4+ and phosphorus may be delivered to the lake in low quantities. OWTS are not believed to be contributing nutrients in excess to the TMDL, a future groundwater flow model should be conducted to quantify the results.