State study finds elevated alluvial groundwater at San Juan Mine but calls CCB risk low given current controls and long travel times

The San Juan Mine Remediation and Restoration Study, prepared by Daniel B. Stephens & Associates and presented to the Radioactive & Hazardous Materials interim committee, found elevated concentrations of total dissolved solids and sulfate in the mine’s alluvial groundwater and identified backfilled coal combustion byproducts (CCBs) as a matter for continued monitoring and management. The study concluded that, under current conditions and planned reclamation, CCBs present a low risk of rapid contamination to down‑gradient receptors because most CCB leachate concentrations are similar to native groundwater and travel times through the bedrock toward the San Juan River are long.

The study matters to residents, downstream water users and land managers because water‑bearing units in the mine area include a shallow alluvial system and two lower bedrock units (Fruitland Formation and Picture Cliff/Picturecliff Sandstone) with much lower permeability. Investigators compiled monitoring data dating to the 1970s, added 2024 sampling, and used time‑series plots, box‑and‑whisker summaries and groundwater potentiometric maps to characterize conditions and flow directions. The report notes the alluvial groundwater generally flows toward the San Juan River and that the greatest constituent concentrations for TDS and sulfate were found in alluvial wells; boron was highest in fill/backfill wells.

Key findings and recommendations

- Groundwater quality: Alluvial groundwater shows the highest constituent concentrations observed in the study (TDS and sulfate), with some values elevated above typical standards for potable use. Surface water samples were generally lower in constituents than groundwater.

- CCBs and travel time: The report says coal combustion byproduct leachate chemistry is often comparable to native groundwater. In lower‑permeability bedrock the modeled travel times to the San Juan River are long (hundreds to thousands of years), reducing the flux and short‑term risk to the river.

- Engineered controls: The San Juan Generating Station and mine operate existing recovery trenches (Memorial Trench Recovery System and Shumway Recovery Trench) that intercept and pump back poor‑quality alluvial groundwater. Stormwater impoundments at the mine are designed to contain a 100‑year storm and infrequent discharges reach NPDES outfalls.

- Reclamation and schedule: The mine operator was reported to be backfilling the Juniper Pit with an anticipated completion date around 2026. The study and presenters said reclamation will rely on backfill, regrading, cover and revegetation; revegetation success is expected to require roughly 10–15 years. The study recommends continued operation of the Shumway Recovery Trench during closure to contain potential shallow alluvial issues.

- Data gaps: Investigators recommended installing additional monitoring wells and establishing improved background well(s) to better distinguish natural background concentrations from impacts related to mining or the generating station.

Committee questions focused on the timeline for reclamation, ownership of surface and mineral estates, post‑mining land use and oversight of contracted studies. Presenters told the committee that land ownership is mixed (federal agencies such as the Bureau of Land Management, private owners and some Westmoreland surface rights) and that post‑reclamation uses identified in permits include grazing and wildlife habitat; solar development has occurred nearby on separate parcels. Presenters also said MMD/NMED review third‑party contractor work and retain the right to sample and validate data during oversight.

Context and next steps

The committee was briefed that historical data are extensive but that a new background well would improve baseline comparisons. The presenters flagged continuing collaboration between MMD and NMED on background monitoring and said MMD holds regular inspections of mine activity. The report’s authors recommended maintaining recovery systems and expanding monitoring to reduce uncertainty as the operator completes backfill and begins long‑term reclamation.

The presentation produced follow‑up requests by legislators for additional technical documentation and clarification about sources of water used for revegetation, bond amounts and the relationship between stormwater capture, dewatering cessation and long‑term groundwater recovery rates. No formal action or vote occurred at the meeting; the committee requested agency follow‑up on data‑gap implementation and monitoring plans.