by Sasha Vinogradova, University of Washington, Tacoma, winner of the WALPA 2024 Undergraduate Scholarship
Fig. 1 – Satellite photo of Lake Killarney, Federal Way, WA with the location of the mesocosms (Google 2024)
Arsenic contamination in freshwater is a growing problem for local citizens due to pollution from historic arsenic sources such as smelters and herbicides. While many smelter and herbicide activities have been stopped, the remnant arsenic continues to create health concerns due to its carcinogenic properties. Research has found that arsenic can remobilize in lakes from sediments and enter the water column and freshwater organisms consumed by humans. One case of arsenic-contaminated waters and sediments is at Lake Killarney (see fig. 1), previously found to have high arsenic concentrations of 200 µg/L in the water column, exceeding the National Toxics Rule freshwater criterion (Gawel et al., 2014).
Fig. 2 – Mesocosm installation at Lake Killarney
To treat arsenic contamination in lakes, sediment dredging is the most long-lasting remediation option, but comes at a price most communities cannot afford. More cost-friendly measures have been proposed, including an existing phosphorus lake treatment, lanthanum-modified bentonite (LMB). In my research, I and a team from UW Tacoma looked at LMB as a possible arsenic treatment alternative. Previous research has shown that LMB can successfully remove arsenic from the water column due to the similar chemical compositions of arsenic (As) and phosphorus (P). Our goal was to test the efficacy of LMB in field conditions, unlike the previous lab-controlled experiments. We created six one-m. diameter mesocosms to section out small areas from the surface to the bottom of the lake to mimic the real environment. We treated the mesocosms with different concentrations — a low (0.25 kg/m2) and high (1.0 kg/m2) dose of LMB (EutroSORB G) — at Lake Killarney (see fig. 2). From June through September, we collected seven sets of data including the temperature, dissolved oxygen, pH, and conductivity profiles, as well as water samples from multiple depths for As and P uptake analysis.
This summer had a strange weather pattern with a cool wave in August causing mixing in the lake’s stratification. Just like the weather, our results came in mixed. The P concentrations in the bottom of the column of the water should have decreased in the high and low LMB concentration mesocosms, as LMB is a certified phosphorus treatment, but the results were messier. The control mesocosms experienced relatively similar P concentrations as the mesocosms with the low and high doses (see fig. 3 A), indicating that other factors interfered with the sediment and water column composition. As a result, the As concentrations had no statistical difference between the control and the two LMB concentrations. At the end of the experiment, the As concentration outside of the mesocosms was similar to mesocosm concentrations at 19 ug/L. This indicates that mixing may have occurred in the LMB layer on the bottom, lowering the efficacy of LMB. We theorized that slight movement of wind and lab equipment may have created small disturbances that were more catastrophic in the small mesocosms. Further in-field testing of LMB is necessary to find other components that could affect As sequestration. It is also important to know whether As presence in lakes affects the efficacy of LMB as a phosphorus treatment.
Fig. 3 A – Average concentrations of (A) phosphorus (ug/L) in the bottom waters of the mesocosms at Lake Killarney, WA collected throughout the summer season 2024. The dotted lines represent the different concentrations of lanthanum-modified treatment (EutroSORB G) used.
Fig. 3 B – Average concentrations of (B) arsenic (ug/L) in the bottom waters of the mesocosms at Lake Killarney, WA collected throughout the summer season 2024. The dotted lines represent the different concentrations of lanthanum-modified treatment (EutroSORB G) used.
References:
Gawel J.E., Asplund J.A., Burdick S., Miller M., Peterson S.M., Tollefson A., and K. Ziegler. 2014. “Arsenic and lead distribution and mobility in lake sediments in the south-central Puget Sound watershed: The long-term impact of a metal smelter in Ruston, Washington, USA.” The science of the total environment 472:530–537. doi:10.1016/j.scitotenv.2013.11.004
