For the last year, Glorieta Geoscience, Inc. (GGI) staff has been evaluating the groundwater monitoring regimen at the closed El Llano Landfill in Española to determine if the existing monitoring well network and monitoring plan can effectively detect potential releases and trace contamination from the landfill. The landfill closed in the 1990’s, and post-closure care monitoring has been in place since the early aughts. GGI has been conducting monitoring since 2013, and has noticed peculiarities in the local hydrology, geology, and detections (and non-detections!) of contaminants that compelled a more in-depth look at the groundwater monitoring system in place.
The hydrogeology at the Landfill is complicated by a mapped fault system (the Road Fault) that bisects the landfill area. The two most obvious pieces of evidence are a fault plane exposure in an outcrop at the Landfill entrance, and consistently higher water levels in at least one of the wells east of the fault compared to those in other wells. Further, three of the five wells at the site have static water levels above the screened interval, which together with information about their completion suggests that these wells could be completed into a deeper, confined or partially confined aquifer. Such wells would not be appropriate for detection monitoring. Analyses of water level trends and water chemistry in the wells support the conclusion that only two of the five existing monitoring wells at the Landfill are completed into the shallow unconfined aquifer and meet monitoring requirements for landfills.
GGI also examined trends in concentrations of certain metals to determine whether or not they are naturally-occurring. As an outgrowth of this study, GGI discovered that uranium concentrations in the two water-table wells increase as groundwater elevation increases. Concentration and water level data from both wells fall on the same regression line, suggesting that uranium concentrations fluctuate with water levels in a consistent manner in both wells. This pattern is not present in the other wells, further supporting the hypothesis that the water-table wells not only intersect the uppermost aquifer (the “water-table”), but also monitor the same aquifer. Because both wells are also located downgradient from the Landfill they are appropriate for detection monitoring. Data from the remaining monitoring wells, due to their completion into a deeper aquifer, do not provide information useful for Landfill monitoring, and these wells will therefore no longer be sampled.
GGI used this information as part of a successful petition to the New Mexico Environment Department (NMED) to amend the Landfill’s Closure and Post-Closure Care Plan so that only the two wells appropriate for detection monitoring are monitored. GGI also analyzed landfill records, groundwater chemistry, and monitoring history and trends to support other requests in the petition reduce monitoring frequency and the analyte suite for groundwater samples. NMED approved the petition, saving the City of Española thousands of dollars each year while more effectively monitoring the landfill for releases of anthropogenic contaminants. The City can now focus its efforts on a reuse plan for the closed landfill, transforming the Landfill from an environmental and financial liability to a community recreational and open space asset.