Groundwater impact assessment will screen likely nitrate contamination from septic systems; pathogens omitted because of data gaps

Sacramento State's Office of Water Programs (OWP) described a methodological framework developed for the Wastewater Needs Assessment to screen where on-site wastewater treatment systems (OTS) may be contributing nitrate contamination to groundwater and to prioritize areas for further investigation or potential remediation.

Khalil Lezajic, research engineer with OWP, said the Groundwater Impact Assessment (GIA) is a first-pass, screening tool "designed to assign likelihood categories for groundwater contamination based on measured data, modeled loading estimates of nitrates, and environmental site attributes." The GIA focuses on nitrate because, Lezajic said, "we had hoped to include pathogens in our analysis, but due to data gaps, this assessment will focus solely on nitrates."

The assessment comprises three components: (1) assessment applicability (geographic scope limited to unsewered, OTS-dependent areas); (2) key-factor selection (measured groundwater nitrate concentrations, modeled nitrate loading estimates from OTS, and site attributes such as soil drainage and depth to groundwater); and (3) likelihood analysis and classification (a three-tier screening that assigns parcels a qualitative contamination-likelihood level).

Modeled loading estimates will combine parcel-level proxies for population (bedroom-count-based occupancy), per-capita nitrogen contributions (project team proposed 13.3 grams N per person per day), parcel area (which affects dilution/dispersion), and annual precipitation. Site-attribute variables include soil drainage classes (from NRCS data) and depth to groundwater; both affect whether nitrates leach quickly to aquifers or are attenuated in the vadose zone. The contractors will use the State Water Board's GAMA well data (measured nitrate concentrations) where available as the strongest indicator.

Lezajic explained how the screening is tiered: parcels with measured nitrate above the 10 mg/L drinking-water maximum contaminant level (MCL) would be flagged as "very high" likelihood; parcels lacking measured data but with modeled loadings exceeding thresholds would be flagged "high"; parcels with no measured/model evidence but with site attributes (shallow groundwater, well-drained soils) suggesting vulnerability would be "moderate"; and others would be "low." The team said the output is intended to guide further site-level investigation rather than to assign blame or regulatory action.

Advisory-group members suggested additional data sources and caveats: distinguish OTS contributions from agricultural or other non-residential nitrate sources in regions with heavy fertilizer or manure use; consider proxies for pathogens or total suspended solids where pathogen sampling is inadequate; and incorporate local groundwater recharge and groundwater-management (SGMA) considerations. Presenters acknowledged these points and said some land-use and source-separation approaches will be considered as the analysis is refined.

Ending: The methodology will be refined iteratively as the team acquires better proxy data for unsewered areas (the UMass model), defines high-density OTS clusters, and sets co-occurrence criteria for site attributes. The contractors sought advisory-group input on additional factors, data sources and how results should support local decision making and prioritization.