Dry Wash PFAS and Heavy Metals Evaluations presentation
WCWCD Info re Dredging Gunlock Reservoir
Mayor Hart indicated that there are a few pieces of information that we've gleaned over the last two weeks that we will share with you. One is that the Washington County Water Conservancy District has hired the engineering firm of Stantec engineering, to be their consultant through the entire reuse process. Stantec engineering is a nationally and internationally recognized firm when it comes to reuse systems, both design and overseeing construction, and have done those systems, many of them across the nation and elsewhere in the world. With that consultant coming on, and I've had the opportunity to meet with three of their representatives at our last District board meeting. They have experts in-house or will find experts to review the situations having to do with the residue coming from water treated to the required level, what the contaminants are, what the concerns might be for the accumulation of those over time, transfer of those by dust and similar. In addition, they have done many, many systems and are going to put together a list of those that exists in residential settings, which is one of the other concerns. We will be seeing information that they're going to provide in the future, responding to those particular concerns. I'm going have Chuck Gillette, who has done some additional work on the water quality issues that were previously discussed and provide that information for this meeting. Council Member Scott inquired as to the timeframe for the engineering firms involvement. Mayor Hart commented that the engineering firm know that the District is in a hurry but anticipate addressing concerns in the next couple of months.
Chuck Gillette stated that the last time he was explaining this in the prior meeting, he ran out of time discussing the accumulation of residues and chemicals, specifically PFAS, in the reservoir sediments. Since that time, he conducted further mass balancing calculations based on the concentration of PFAS in wastewater. PFAS is a dissolved acid that does not settle well into sediments. He considered a hypothetical scenario where PFAS settles into the dirt, evaporate, and then increases in concentration over time. He then assessed how much PFAS could be blown into the air as dust. To evaluate that, he compared potential PFAS dust concentrations to current dust levels at Black Desert Resort, monitored by Purple Air. On the worst recorded day, dust levels were 39 micrograms per cubic meter (PM 2.5). Michigan's air quality standard for PFOS and PFOA is 0.07 micrograms per cubic meter. This means no more than 1.8 micrograms per kilogram of dust could be PFAS. Assuming soil density at 100 pounds per cubic foot and PFAS concentration at 13.5 parts per trillion, it would take 608 million years to reach harmful levels. Even if 50% of PFAS settled, it would take 20,000 years to exceed the standard, assuming there was no additional sediment.
The air quality standard is based on continuous exposure, but dust events are sporadic. He used the worst dust day at Black Desert Resort for the. Regarding arsenic, it enters the reservoir at 0.0044 milligrams per liter. Assuming 50% settles in the top five millimeters of sediment, and using OSHA's standard of 39 micrograms per cubic meter of air, 5.43 kilograms of arsenic enter annually. If half settles, it results in 5.2 milligrams per kilogram of sediment. Reaching hazardous airborne levels would take 49,000 years, assuming no additional sediment and conservatively high arsenic estimates. Based on his calculations, the risks appear to be very low but he will await further information from Stantec Engineering who may provide actual research data, empirical evidence, and testing results that could offer different perspectives. For now, using the available data and math, he found it difficult to see how the substances could pose a concern in the sediment or water.
Mayor Hart referred to a suggestion made at the last Work Meeting regarding dredging Gunlock Reservoir and the suggestion that building it up might meet additional water requirements without constructing Dry Wash Reservoir. He asked the Washington County Water Conservancy District for an explanation on whether that approach would be feasible and more cost-effective. They have provided a response, a copy of which is available as an attachment to this agenda.