by Jeffrey Tepper (University of Puget Sound), Kyle Steelhammer (Northwest Aquatic Management, LLC) and Don Russell (City of Lakewood Lake Monitoring Program)
Fig. 1 – Lake Steilacoom showing inflow streams Clover Creek (CvC) and Ponce de Leon Creek (PdL) and the outflow at Chambers Creek (ChC). For Phase One, ZVI was applied to groundwater vents (red circles) and hyporheic zones (red rectangles).
Over the past several decades, many lakes in the Puget Sound region have experienced an increase in the frequency and severity of hazardous algal blooms (HABs). Lake Steilacoom, located in Lakewood, Washington, is one such lake. Every year between 1998, when recordkeeping began, and 2017, when county funding for monitoring ended, the lake experienced at least one HAB advisory related to high Microcystis levels, some lasting as long as 17 weeks. The approach to treating these HABs in recent years was to apply an algicide, which was effective in the short term because it killed the algae. However, decomposition of these algae released nutrients that then fed the next bloom, therefore requiring, in some years, three costly algicide treatments. In 2024 the Lake Steilacoom property owners association chose to try an alternative approach using zero valent iron (ZVI). The purpose of this article is to summarize the details of that treatment plan and its results, and to suggest that ZVI can be an effective, economical, and environmentally friendly means of controlling HABs in lakes like Steilacoom.
About the lake
Lake Steilacoom (see fig. 1) was created in the 1850s when its outflow, Chambers Creek, was dammed to power a sawmill. Today the lake has an area of 320 acres, with an average depth of ~11 feet and a maximum of ~20 feet. Its main water sources are groundwater vents and two groundwater-fed streams, Ponce de Leon Creek and Clover Creek. Because these water sources are in the south basin of the lake and the outflow is at the north end, water travels through the lake from south to north with a residence time of about six months. A significant share of the groundwater entering Lake Steilacoom originates from Gravelly Lake to the south, which is hydraulically upgradient and separated by only ~1500’ of permeable glacial outwash sands and gravels. Gravelly Lake becomes strongly stratified during summer and fall, when the soluble reactive phosphorus content (SRP) of the hypolimnion can exceed 300 ppb. As a result, the SRP content of groundwater entering Lake Steilacoom is high (averaging 30-40 ppb) and this is a main source of nutrients for HABs.
How ZVI works
ZVI is simply metallic iron, which is commercially available in a range of grain sizes (10 – 300+ μm). Depending on size, ZVI can be applied in dry form or as a slurry, and within hours of contacting water the particles will develop a highly reactive coating of iron oxyhydroxides (see fig. 2). Phosphorus (P) and other ions in solution will bind to this coating and remain sequestered as long as conditions are not strongly reducing (i.e., hydrogen sulfide producing). The capacity of ZVI to bind P varies with grain size, but the range (10 – 100 mg P / g ZVI) is comparable or greater than that of synthetic P sequestration products.
Treatment plan
Recognizing that external loading, not sediment release, is the main source of P and nitrate (NO3) to Steilacoom, and that the lake has a short residence time, we developed a two-phase treatment plan. The goal of the first phase was to intercept and sequester P and NO3 at the main points where they enter the lake. This entailed applying ZVI at four groundwater inflow locations (see fig. 1): two springs on the bottom and the hyporheic zones at the mouths of Ponce de Leon and Clover Creeks. A total of 5,500 lbs. of coarse (325 μm) ZVI was spread in dry form over three acres, which translates to 100g/m2 and could in theory sequester 5 g of P/m2. By applying ZVI early in the spring, we hoped to reduce or even eliminate the need to treat the whole lake later in the summer or fall. The goal of Phase Two, which we implemented in mid-August, was to prevent an impending bloom by stripping P and NO3 from the water column. We calculated that this would require application of 14,000 lbs. of fine-grained (44 μm) ZVI, based on the lake’s volume, an assumed SRP content of 40 ppb, and our experimentally determined ZVI adsorption capacity of 30 mg P/g.
Results
Comparison of samples from treated and untreated areas of a groundwater vent showed that water passing through the ZVI bed had lower SRP (<4 ppb vs. 23 ppb) and lower NO3 (<0.1 ppm vs. 0.74 ppm). These results were obtained five months after the ZVI was initially applied, indicating that the iron remained effective well past the date of application. We also assessed the effectiveness of Phase One by looking at total phosphorus (TP) in lake water samples taken from both basins of the lake in late summer. Compared to the average August-October levels for 2004-2018 (TP = 38 ppb), this year’s values were 20-40% lower (30 ppb in north basin, 22 ppb in south basin), a decrease we attribute to having successfully sequestered a portion of the incoming SRP.
Nonetheless, by mid-July the Secchi depth had shallowed to ~2.5m and minor Microcystis was observed under the microscope, so we elected to “play it safe” and carry out Phase Two. The entire lake was first treated with an algicide to kill the algae. We waited several weeks for those algae to decay and release their nutrients, and then in mid-August we applied the fine ZVI, this time as a slurry. Water samples collected two weeks after this treatment showed that both SRP and NO3 were below detection in all samples but one.
Fig. 2 – Scanning electron microscope images of ZVI particles before (left image) and after exposure to water. The oxidized Fe crust visible on grains that were exposed to water is where the P is bound.
Conclusions
We are now well into November and there has been no HAB at Lake Steilacoom. Small Microcystis scums have been noted at a few locations, but nothing comparable to the prolonged HABs that have occurred this year at hydrologically similar lakes nearby. We attribute this success to having kept SRP levels below ~20 ppb all summer, thereby denying algae the nutrients required for a bloom. In addition, the switch to ZVI reduced the cost of HAB mitigation by 20-45% relative to the previous approach, saving the residents of Lake Steilacoom tens of thousands of dollars. These savings are mostly because of a 50-66% decrease in the use of expensive synthetic algicides. The property owners have elected to use ZVI again next year, so we will continue the experiment in 2025, looking to see, among other things, whether we can discern in water chemistry any residual benefits from the 2024 ZVI applications.
ZVI is not a new tool; it has been used for over 130 years to remediate contaminated groundwater. Its application to lakes has been much more limited, largely because of concerns that under anoxic conditions iron will dissolve and release the P bound to its surfaces. However, for shallow, flow-through lakes like Steilacoom, where external loading is the main source of nutrients, ZVI can be an effective, economical, and natural alternative to synthetic P sequestration products.
