Effects of Sulfate on Wild Rice
Scientists at the University of Minnesota have recently learned a great deal about the effects of sulfate on wild rice and on mercury methylation.
John Pastor, Ph.D., has studied the ecology of wild rice for more than a decade. His controlled experiments with wild rice in outdoor mesocosms (large tanks) demonstrated that increased sulfate in surface waters impairs wild rice. The abstract of Dr. Pastor’s 2017 publication on wild rice and sulfate explains,
In outdoor mesocosms, sulfate additions to overlying water increased sulfide production in sediments. Wild rice seedling emergence, seedling survival, biomass growth, viable seed production, and seed mass all declined with sulfate additions and hence sulfide concentrations in sediment. These declines grew steeper during the course of the 5 yr of the mesocosm experiment.
Dr. Pastor’s work, and that of his masters and doctoral degree candidate, Sophia LaFond-Hudson, demonstrated that the addition of high levels of sediment iron to tanks or buckets with high sulfate levels resulted in blackened roots (see Figure 1 above) and harmed wild rice.
Ms. LaFond-Hudson’s research, found that during seed production, in buckets of water with 300 mg/L of sulfate, iron sulfide rapidly accumulated on wild rice roots and accumulated up to 100 times more sulfide than roots in low-sulfate control water. Ms. LaFond-Hudson’s 2018 publication in Biogeochemistry explained the geochemical process resulting in fewer and lighter seeds in wild rice plants.
Dr. Pastor summarized the mesocosm research in his expert opinion provided to the Administrative Law Judge in support of preserving Minnesota’s wild rice sulfate standard:
The MPCA assumes that any precipitation of sulfide by iron helps to protect wild rice. Our experimental mesocosm research has substantially undermined this assumption.
We learned that iron sulfide precipitates rapidly on wild rice roots in midsummer at the time when the plants are beginning to flower and take up additional nutrients for the ripening seeds. The iron sulfide precipitates gave the roots a black appearance, compared to amber or rust colored roots on healthy plants exposed to sulfate concentrations near the current fixed standard of 10 mg/L. Seed nitrogen, seed count and seed weight were all markedly reduced in plants with back root surfaces exposed to high sulfate surface water concentrations (300 mg/L) because these black iron sulfide precipitates inhibit the uptake of nutrients necessary for the filling and ripening of seeds necessary for propagation of wild rice.
Dr. Pastor’s Expert Opinion strongly recommended preserving Minnesota’s wild rice sulfate standard:
[B]ased on my training and experience, it is my opinion that the weight of the scientific evidence supports retaining Minnesota’s existing sulfate standard of 10 mg/L to protect wild rice.
Based on current scientific evidence, an equation determining “protective” sulfate levels based on iron in sediments and available carbon is not a defensible strategy to protect wild rice.
[S]ulfate concentrations an order of magnitude or more above 10 mg/L, as would be allowed in some water bodies by MPCA’s proposed flexible standard, are likely to result in decline and extinction of wild rice over time.
Administrative Law Judge, LauraSue Schlatter, disapproved the MPCA’s proposed rule to repeal Minnesota’s wild rice sulfate standard, citing Dr. Pastor’s research in concluding that the equation proposed by the MPCA was not equally or more protective of wild rice than Minnesota’s existing standard:.
For the reasons set forth in the following section regarding the equation-based standard, the Administrative Law Judge further concludes that the MPCA has not presented facts adequate to support the reasonableness of the proposed repeal of the 10 mg/L sulfate standard without a replacement standard that is equally or more protective of wild rice waters. Therefore, the proposed rule repealing the 10 mg/L sulfate standard is defective because it violates Minn. R. 1400.2100.B.
With one notable exception, the MPCA responded to each of the arguments raised by the commenters with arguments that were supported by peer-reviewed research. . .
The exception, for which the MPCA did not offer a convincing response, was raised by several parties, most notably Dr. John Pastor, one of the scientists on whose foundational research the MPCA relied for its conclusions that sulfide, rather than sulfate, is the direct cause of damage to naturally-occurring wild rice. Dr. Pastor’s continuing mesocosm research has indicated that, while increased iron may counter the toxicity of sulfide to wild rice seedlings in the springtime, iron sulfide plaques form and precipitate on the plants’ roots during the flowering and seed production phases of the wild rice life cycle. These plaques result in fewer and smaller seeds, with reduced nitrogen content, leading to extinction of the wild rice plant within 4 or 5 years at about 300 mg/L of sulfate, and greatly reducing wild rice plant population viability at lower concentrations of sulfate. Dr. Pastor hypothesizes that this occurs because the increased plaque appears to block uptake by the plant of nitrogen during the critical flowering and seed production portion of its life cycle.
Mathematics Professor Emeritus, Joel Roberts, Ph.D., also analyzed the MPCA’s proposed equation that would replace Minnesota’s 10 mg/L wild rice sulfate standard. He pointed out that the formula proposed by the MPCA was even less protective than the 2015 proposal he had previously criticized. Dr. Roberts summarized that the formula “is inadequate for explaining the data from the Wild Rice Field Study” and “inadequate for protecting Minnesota’s Wild Rice.”
READ Expert Opinion of John Pastor, Ph.D., regarding effects of sulfate on wild rice, Nov. 2017, and supporting attachments, including his article, Sophia LaFond-Hudson research and spreadsheet wild rice data.
Effects of Sulfate on Mercury Contamination and Eutrophication
Recent research at the University of Minnesota, led by Amy Myrbo, Ph.D., has demonstrated that high levels of sulfate in surface water not only impairs wild rice, but increases mercury, methylmercury, and nutrients (nitrogen and phosphorus) that can result in eutrophication of lakes.
Dr. Myrbo found that sulfate loading in mesocosms both increased release of inorganic mercury from sediment into the water and increased the proportion of mercury that was converted to toxic methylmercury. Sulfate increased the release of inorganic mercury from sediments to the water by as much as 2.2 times over the low-sulfate control.
With sulfate levels of either 100 mg/L or 300 mg/L, methylmercury increased 5.9 times as compared to the control experiment where no sulfate was added.
The plain English summary of Dr. Myrbo’s article summarized:
This study demonstrates that adding sulfate to a wetland can not only produce toxic levels of sulﬁde but also increase the surface water concentrations of nitrogen, phosphorus, mercury, and methylmercury.
READ A. Myrbo, et al. Increase in Nutrients, Mercury, and Methylmercury as a Consequence of Elevated Sulfate Reduction to Sulﬁde in Experimental Wetland Mesocosms, J. of Geophysical Research: Biogeosciences (2017)
Background – Scientists explained that Minnesota’s Existing Sulfate Limit is Needed and Reasonable and that the MPCA’s Proposed Equation Would Not Protect Wild Rice.
Long before the Minnesota Pollution Control Agency (MPCA) proposed its 2017 rule to repeal Minnesota’s wild rice sulfate standard and replace it with an equation, scientists had explained that the MPCA’s approach was unreasonable and would not protect wild rice.
Dr. John Pastor presented his research, including a 2016 Annual Report from his research demonstrating that allowing higher levels of sulfate pollution in waters that have high levels of iron would harm wild rice.
During the course of these experiments, wild rice roots in tanks with more than 50 mg/L sulfate had become blackened. In contrast, plants grown in the low sulfate treatments had orange stains on the roots throughout the annual life cycle (Fig. 1). Using SEM elemental scans, we identified the black plaques as iron sulfide (FeS) plaques whereas the orange stains had iron but no sulfide and are most likely iron (hydr)oxides. . .
After two years, sulfate amendments had the greatest effect, reducing production as in the first experiment regardless of iron amendment . . . While we cannot yet conclude from this experiment that iron has a strong depressive effect on wild rice growth via FeS plaques on roots, we can conclude that iron has no beneficial effect by reducing the toxicity of sulfide.
Plants with the black FeS plaques on their roots produced fewer and less viable seeds, perhaps because the plaques potentially impair the uptake of phosphorus and nitrogen.
Comparing observed sulfide concentrations in the MPCA field data with predicted sulfide concentrations obtained by applying the MPCA’s equation demonstrated to me the poor predictive power of the proposed equation. The lack of consistency in the ratios of predicted and observed sulfide provides no confidence that the MPCA’s Proposal will provide a reliable prediction of sulfide levels. . . the MPCA Proposal seems like an unreliable method to protect wild rice from excess sulfide.
READ the November 2015 technical opinion of international expert on structural equations, Bill Shipley, Ph.D., challenging use of MPCA’s formula to predict levels of sulfate that would protect wild rice.
In summary . . I am not convinced that the correct SEM (i.e. the one in Figure 7) was actually tested. In other words, even if the prediction equation had good “within-sample” predictive ability (it doesn’t), we could not know if we could apply this equation to new observations.
I believe that a new standard is not the default position, but that the existing standard is the default position . . . 1) the proposal redefines “protect wild rice” into a much weaker sense than that of the existing standard . . . The draft proposal would allow any of the non-zero-density stands to be potentially reduced in abundance through elevated SO4-2 [sulfate] release if they have enough reactive Fe [iron] and not too much organic C [carbon].
[Dr. Pastor] said his research undercuts the MPCA’s theory that higher concentrations of iron in water protect wild rice because they reduce sulﬁdes. He said he’s found that much of the iron sulﬁde that precipitates out of the water forms plaques on the roots of wild rice plants that hamper their ability to produce seeds. . . So he said it’s premature for the MPCA to conclude that its model for the role of iron is correct. Established science shows that the existing 10 parts-per-million sulfate standard protects wild rice, he said, so the safest course is to stick with that.
Paula Maccabee, attorney for the environmental group WaterLegacy, said the PCA appears to be ignoring new research that shows iron may not play any beneﬁcial role in reducing the impact of sulfate. That research, by Prof. John Pastor at the University of Minnesota Duluth and funded by Sea Grant, concluded there is no buffering impact by iron on the sulfate/sulﬁde conversion.
“We already know that the existing sulfate limit is effective and reasonable. But PCA, ignoring evidence that the equation they are developing is ﬂawed, continues down this path and away from protecting wild rice,” Maccabee said. “They are developing this very complex and ﬂawed process based on pressure from the mining industry and Iron Range lawmakers when their primary concern should be protecting wild rice.”
Basic Facts about Effects of Sulfate Pollution
- In addition to destroying wild rice, sulfate increases mercury methylation and toxic methylmercury contamination of fish. Methylmercury is a potent neurotoxic, harming the developing brains of fetuses, infants and children in Minnesota.
- Sulfate pollution can also release phosphorus from sediments, leading to eutrophication (algae blooms) in once-clear lakes.
- READ the Expert Opinion of Duluth child psychiatrist, Dr. Margaret Saracino, describing the health risks to fetuses, infants and children from exposure to methylmercury in fish.
- READ WaterLegacy’s Fact Sheet Protecting Minnesota Clean Water and Wild Rice from Sulfate Pollution.
Citizen Scientists Document Wild Rice Impairments Downstream of Mine Pollution (2010)
In the autumn of 2010, biology teacher and experienced hunter, fisher, and ricer, Len Anderson and other observers documented the plentiful, healthy natural wild rice beds in the Partridge River upstream compared with downstream from decades of mine wastewater discharges.