County staff says Aspen Creek infiltration basins likely can handle about 500,000 gallons per day; NDEP requests more review

Lyon County staff and the county's engineer presented updated field testing, groundwater monitoring and preliminary modeling for the Aspen Creek rapid infiltration basins on May 15, reporting a best current estimate of roughly 500,000 gallons per day of effluent disposal capacity.

The presentation, led by county wastewater staff and engineer representatives from Dowell, summarized months of slug tests, constant-head tests, noninvasive seismic (REMI) surveys, excavation observations and installation of additional piezometers and monitoring wells. Dowell told the board its review of the expanded dataset produced a preliminary capacity estimate materially higher than an earlier engineering estimate but lower than initial design targets.

County presenter David Brickhead (wastewater operations) and Dowell's consultants described key findings: groundwater mounding under the basins is more pronounced at the north end of the cells, tests produced a wide range of hydraulic conductivity values (which drive disposal capacity calculations), and trenches and a donated 14-inch pipe installation to move water toward the south side produced limited improvement. Field excavation found variable groundwater depths across the basins and no clear geologic obstruction, the presenters said.

Why it matters: The Aspen Creek basins were originally designed for 1.4 million gallons per day. After startup, actual performance was significantly lower, and the limit on disposal capacity has implications for local development approvals that rely on sewer capacity and for seasonal operations (storage, irrigation reuse and temporary trucking). County staff said decisions about will-serve commitments and final map approvals depend on validated capacity.

What the county said it has done: Dowell performed slug and constant-head hydraulic testing; the county installed continuous piezometer monitoring in regulatory wells and in multiple locations inside each basin; geotechnical REMI surveys were run to seek subsurface heterogeneity; trenches and piping were trialed to improve internal flow; and additional push-bores and monitoring wells were installed on county and adjacent private property to better define groundwater response. The county also submitted a plan to NDEP for ‘‘assessing and enhancing effluent disposal capacity' and a reclaimed-water dust-control plan for operational flexibility.

The preliminary engineering memo from Dowell cited in the presentation estimates Aspen Creek capacity at about 500,000 gallons per day (higher than an earlier 350,000 gpd estimate but well below the 1.4 million gpd design). County staff said they have submitted the memo and supporting data to the Nevada Division of Environmental Protection (NDEP) and requested a technical meeting.

NDEP response and next steps: County staff said NDEP has asked for further evidence and has not yet given regulatory approval to increase available capacity for development approvals. NDEP raised two issues in recent conversations recorded by staff: (1) a water-quality incident earlier in winter (temporary elevated nitrogen) that the county reported and addressed, and (2) NDEP wants a more complete, peer-reviewed engineering assessment before changing permit conditions or approving additional will-serve allocations. The county plans to build a calibrated three-dimensional groundwater model using the enlarged monitoring dataset, continue optimized dosing and stress tests (including cooperation with the local golf course to provide additional irrigation area for test discharges), and seek a technical meeting with NDEP to review the modeling approach and conclusions.

Public comments: Representatives of several local developers and the project's development manager asked the board to confirm how much permitted capacity is already allocated, whether will-serve letters will be converted to certificates of occupancy upon final inspection, and for more precise timelines for long-term solutions (e.g., major permit modifications, trucking to alternate plants, additional storage). Developers said uncertainty about available capacity affects ongoing projects and requested transparency about current commitments.

Board direction: Commissioners did not adopt any regulatory change at the meeting. Staff recommended continued monitoring and model development, and the board asked staff to continue engagement with NDEP and to return with additional analysis. County staff said a major permit modification (to enable trucking effluent to other plants) remains an option but would take months because it requires public review.

Ending: County staff stressed the lengthy, iterative path ahead: more real-time monitoring, completion of a calibrated groundwater model, and a follow-up technical meeting with NDEP. For now the county's working best estimate is 'about 500,000 gpd' pending the regulator's review and any model changes.