We know that weather influences water quality in Iowa’s rivers. Last year, there was a drought and nitrate was lower than usual. This spring, it’s been wetter and nitrate is higher than usual. If you monitor for 10 years and the first 5 are a little wetter or drier than the last five, you’ll a water quality trend to go with it. Boring!
What we really want to know is how people are influencing water quality. We can get a lot closer to that answer by peeling away the obvious weather-related patterns to reveal underlying trends.
In statistics, it’s called a covariate or an explanatory variable. If there’s a relationship between your water quality metric and some other thing you’re not really interested in (i.e. streamflow), you can model that relationship to account for part of a water quality trend over time. What’s left over might be the things you’re really interested in (i.e. how water quality has been affected by changes in crop rotations, conservation practices, sewage treatment, manure management, and drainage). It’s common enough in the scientific literature (Robert Hirsch’s Weighted Regression on Time, Discharge, and Season is a good example), but should be used more often for progress tracking at the watershed scale.
To illustrate this general approach, I downloaded daily nitrate data from three stations maintained by the US Geologic Survey. The sensors at the Turkey River at Garber and the Cedar River near Palo (north of Cedar Rapids) were installed in late 2012; the sensor Raccoon River near Jefferson was installed in 2008. I wanted a high frequency dataset (to minimize sampling error) that included the episodes of “weather whiplash” in 2013 and 2022.
“Residuals” are the difference between what we predict and what we measured. In the first panel, that’s the difference between a measurement and the long-term average. In the second and third panels, we see how nitrate measurements differ from what we’d expect given flow in the stream today, and flow in the stream last year. Gray dots – daily measurements. Red dots- yearly averages. Blue dotted line – trend. If I did this right, some of the dots should get closer to the middle.
Nitrate concentrations in rivers increase as the weather gets wetter and streamflow increases… up to a point. When rivers are running very high, there’s a dilution effect and nitrate concentrations fall. Based on that relationship, we can explain high nitrate levels in the Cedar River in 2016 (a wet year) and low nitrate levels in 2021 (a dry year).
Nitrate concentrations tends to be highest on wet spring days following a dry summer and fall, as nitrate that accumulated in the soil during the drought is flushed into drainage systems or washed off the land surface and into rivers. Here I’ve calculated a moving average of flow over the previous 365 days, and compared that to nitrate concentrations during high flow or low flow conditions. Based on that relationship, we can explain high nitrate in the Cedar River on wet days in the spring of 2013 and 2022 (following a dry year) and low nitrate on wet days in the spring of 2019 (following a wet year).
After making these adjustments, the downward trend in the Cedar River looks much smaller (0.53 mg/L per year, adjusted to 0.25) and is overtaken by the Turkey River (0.37 mg/L, adjusted to 0.28). The adjusted trends are statistically significant and could be attributed to conservation efforts in those watersheds.
How did I do this? For technical details, read here.
However, there’s still some weather-related patterns we haven’t accounted for. The Raccoon River near Jefferson also had a steep decline in nitrate since 2013 (1.42 mg/L per year, adjusted to 0.77 mg/L per year) but if you look at the entire record (going back to 2008), it’s part of an up-and-down cycle. I’ve seen that same pattern in the South Skunk River. The model explains some of those swings but doesn’t fully explain high nitrate in fall of 2014, spring of 2015 and spring of 2016. Perhaps the nitrogen that accumulated in the soil during the drought of 2012 took several years to flush out.
In addition to streamflow and last year’s weather (antecedent moisture is the technical term), nitrate can be explained by season, soybean acreage, and baseflow. If it’s not enough to know that water quality is improving or getting worse, and you’d also like to know why, then let’s peel that onion!
Later this month, we are releasing a report with the findings from Story County’s 2021 water monitoring season.
In some ways, 2021 was an unlucky year to launch a water quality monitoring program. Story County was in drought conditions for much of the year, and smaller streams were frequently dry when we did our monitoring routes.
In some ways, it was an ideal year to launch a monitoring program, because weather always has an influence of water quality and the challenging conditions in 2021 forced us to better account for it.
For the report, this means asking a simple question: “was there enough water to float a canoe on the day you sampled?”
When the South Skunk River is too low for paddling:
Not much water (and not much nitrogen and phosphorus) reaches the Gulf
“Hot spots” for nitrogen and phosphorus are below wastewater treatment plants
When the South Skunk River rises high enough for paddling:
“Hot spots” for nitrogen are in the Headwaters of the South Skunk River Watershed upstream of Ames, as shown in the graph
E. coli levels upstream of Ames (and Ioway Creek) get worse but still meet the standard
E. coli levels downstream of Ames (and Ioway Creek) get better, but still exceed the standard
If I had less curiosity and more sense, I would have written a short report: “great job everyone! We collected a lot of data. Here it is! It’s possible that drought had an influence on water quality.” This was more work, but I hope you get more out of it.
Before state wastewater standards went into effect in the 1960s, raw sewage could flow directly to a stream without treatment. Despite the standards, this continues in many areas today. In areas called “unsewered communities,” outdated or poorly functioning septic tanks still allow untreated wastewater into our waters. The Iowa DNR works with these communities to find funding sources and alternative treatment systems and to allow adequate time to upgrade the systems.
The Governor has announced that additional funding through the infrastructure bill that will be available to help unsewered communities upgrade their systems. Could this make a big difference for water quality in Iowa? Statewide, I’m not sure, but I’ve taken a closer look at the Iowa River Basin upstream of Marshalltown, where we know of 11 unsewered communities. Based on my first look at the data, it appears that these communities have little influence on E. coli in the Iowa River itself, but could make a difference for water quality in tributary streams like Beaver Creek in Hardin County.
A Water Quality Improvement Plan for E. coli bacteria in the Iowa River Basin was released by Iowa DNR in 2017. As required by the Clean Water Act, these kinds of plans include a Total Maximum Daily Load (TMDL) of pollutants that a water body could handle and still meet water quality standards. Author James Hallmark compares this pollution budget to a family budget: regulated point sources are your fixed bills, non-point sources are your variable expenses, and the margin of safety is your emergency fund. I like this analogy and would add that without some understanding of where your discretionary spending is going, and a realistic strategy to reign it in, you’re probably not going to achieve your goals.
The Water Quality Improvement Plan includes a comprehensive list of E. coli sources but doesn’t single any of them out as being particularly important. It includes a list of potential solutions, but it doesn’t identify which of those would make the most difference. That’s a job for a Watershed Management Plan written with stakeholder input, apparently. However, the document is chock-full of load-duration curves, which I wrote about previously. We can use the information in these charts and tables to take the next step and begin to narrow down where and when the pollution is most serious!
In this article, I won’t pay much attention to “High Flows” and “Low Flows” because there wouldn’t be much recreational use under these conditions. I also don’t look at “mid-range” flows because there’s a bigger mix of sources influencing water quality in these conditions. A closer look at the other two categories is revealing.
If houses are discharging raw sewage directly into a stream, we’d expect to see the highest E. coli concentrations when the stream is running lower than normal, and there’s less dilution. This is indeed what we see in Beaver Creek in Hardin County, which is downstream from the unsewered community of Owasa. Beaver Creek would need a 79% reduction in E. coli load to meet the primary contact recreation standard during “Dry Conditions” and a 38% reduction during “Wet Conditions”.
Treated sewage also has the biggest influence when streams are lower than usual. The upper reaches of the South Fork receive effluent from the small towns of Williams and Alden, which have waste stabilization lagoons. It’s likely that some bacteria makes it through the treatment process, and this would explain why E. coli is higher during “Dry Conditions” (needing a 73% reduction) than during “Wet Conditions” (needing a 30% reduction). When their permits come up for renewal, Iowa DNR could require a UV disinfection system to ensure that E. coli in effluent is no greater than 126 colonies/100mL.
In a watershed with few people and many hogs, we’d expect to see the highest E. coli concentrations when the streams are running high and runoff from fields that receive manure application is more likely. This is indeed what we see in Tipton Creek in Hardin County, a watershed containing 47(!) CAFOs, but the levels are not especially high compared to other sites in the Iowa River basin. The recreation standard is met during “Dry Conditions” and would need a 36% reduction during “Wet Conditions.” Handled correctly (applied to flat ground at the right time, and preferably incorporated into the soil), manure and the microbes it contains can be kept out of streams. Preventing loss of the nutrients in manure is a more difficult challenge—nitrate concentrations in Tipton Creek often exceed 20 mg/L!
It’s not clear to me whether primary contact recreational use of these streams is a relevant or attainable goal, or whether we should be calibrating our level of concern to the secondary contact recreation criteria. Unless there’s a permit holder affected, IDNR doesn’t investigate whether there’s enough water for kayaking in Tipton Creek, or whether children play in Beaver Creek, so the designated use is presumptive and tells me nothing.
To protect fishing, paddling, and children’s play on the Iowa River itself, where and when should we focus? The Iowa River at Marshalltown needs a 60% reduction in bacteria load to meet the recreation standard during “Wet Conditions” (10-40% flow exceedance). However, it actually meets the primary contact recreation standard during “Dry Conditions” (60-90% flow exceedance). Focusing on unsewered communities in the watershed would NOT be an effective way to address this impairment.
Galls Creek in Hancock County has some of the worst E. coli levels measured in the basin, and would have a larger per-acre benefit to the Iowa River if standards could be met. Galls Creek has no unsewered communities but at least 20 farmsteads located along the creek that could have issues with septic systems overflowing under wet weather. The watershed has little woodland and no pasture, so land application of manure from the several CAFOs in the watershed would be most likely animal source of E. coli.
This is just a partial review of one of three HUC8s in the Iowa River Basin. There is much more to learn from further discussion with people who know the area well, or from on-site investigation. However, I hope I’ve demonstrated how we might squeeze some more insight out of the data we have, in order to make smart investments in water quality.
Water quality in the South Skunk River is still poor but has gotten better in the last five years. One reason for improvement is a new disinfection system at the Ames wastewater treatment plant.
Dr. Chris Jones recently shared a water quality index he developed for Iowa rivers, combining five important water quality metrics. Of the 45 sites in the Iowa DNR’s ambient monitoring network, the South Skunk River near Cambridge scored “poor” and ranked 34th overall. This site also has the 3rd highest phosphorus and the 5th highest E. coli levels.
A follow-up article looked whether the current (2016-2020) water quality index has changed from previous decades. Most rivers have stayed the same or gotten worse, but the South Skunk had a better score. Three of the metrics (total phosphorus, total nitrogen, and E. coli bacteria) improved, while one (turbidity, a measure of sediment in the water) got worse.
Phosphorus and muddy water usually go together, so this odd pattern demanded some explanation. I did my own analysis of the data, curious if the changes were happening under drier conditions (when wastewater has a bigger influence), wetter conditions (when agricultural runoff has a bigger influence) or both.
See last week’s post for an explanation of the flow categories I’m using. I use a lot of boxplots, which show both the central tendency and the spread of the data. The lower end, middle, and upper ends of the box are the 25th, 50th (median) and 75th percentiles. The “whiskers” show the maximum and minimum, unless they’re really far out there, in which case the “outliers” represented by dots. Water quality data never fits a bell curve, so lopsided boxes and outliers are to be expected.
It turns out that the river didn’t get any muddier (it got less muddy) when you compare wet conditions to wet conditions and mid-range conditions to mid-range conditions.
It’s just that the past five years were a little wetter and so a larger share (43% vs 29%) of the samples were collected during wet conditions when the river was moving swiftly and carrying more sediment. A smaller share (25% vs 42%) of the samples were collected during dry and low-flow conditions when the water is usually clear. That made the average sediment concentration increase.
Weather also can explain trends in phosphorus. In the past five years, a smaller share of the samples were collected during dry and low-flow conditions when phosphorus concentrations are especially high. That made the average phosphorus concentration decrease.
Why is phosphorus so high when it’s dry? The monitoring site in question is just below the outfall of Ames Water Pollution Control Facility on 280th St, about 4 miles north of Cambridge. This facility discharges over 6 million gallons a day of treated wastewater. When conditions are dry, the effluent is less diluted, and so phosphorus in the stream approaches phosphorus levels in the effluent (which averages 3.8 mg/L), as shown in the graph below. This year we’ve also monitored West Indian Creek below the Nevada wastewater treatment plant and have seen the same pattern.
Wastewater treatment plants are regulated to minimize the impact on receiving waters and the Ames WPC Facility has one of the best compliance records in the nation. However, while the treatment process is very good at removing ammonia, solids, and oxygen-depleting substances, the process is not that effective for removing nutrients.
The Iowa Nutrient Reduction Strategy is not voluntary for wastewater treatment plants. The Ames Water and Pollution Control Department commissioned a feasibility study as a condition of its permit, which determined that the facility could achieve a 67% reduction in total nitrogen and a 75% reduction in total phosphorus by replacing its trickling filters with an activated sludge treatment system. This new system will be phased in as the filters reach the end of their useful life (starting in 2027) and will cost $39.6 million.
In the last five years we have not seen any reduction in phosphorus during dry and low-flow conditions when wastewater treatment systems have the biggest influence. However, there was a reduction in phosphorus* under mid-range and high flow conditions! This is really interesting–I’ll have to look at some other data sets to see whether it holds up and whether we can link it to conservation practices in the watershed.
For E. coli, we see improvement across all conditions, with the largest improvement during low-flow conditions* when wastewater has the biggest influence.
The effluent discharged to the river has gotten a lot cleaner since the Ames Water Pollution Control Facility built $2 million disinfection system using ultra-violet (UV) light. The system was completed in March of 2015. During the recreational season (March 15-Nov 15), treated effluent passes through two banks of lights that kill microbes. Prior to this, some E. coli and pathogens could make it through the system and end up in the river. Chlorination, a good solution for disinfecting drinking water and swimming pools, is not ideal for wastewater because it can harm fish.
Kris Evans, an environmental engineer for the City of Ames and the project manager, said this about the system:
“By using UV we continue to be “chemical free” for treatment of the wastewater and it’s much safer for staff since they don’t have to chlorinate and dechlorinate. When flow is low at the plant, we are able to lower the intensity of the bulbs to save energy and still meet permit limits; as flow increases so does the UV light intensity. The City started design of the UV system before it was mandated in a permit; we knew it was coming, but wanted to be proactive in the treatment of water, making it safer for those who recreated in the river. It was also the first project the department funded through an State Revolving Fund (SRF) loan.”
We still see high E. coli levels in other streams that receive effluent, but smaller wastewater treatment systems are also making the switch to comply with new permit requirements. In Story County, Gilbert added a UV disinfection system in 2019, the Squaw Valley HOA completed theirs in 2020, and Nevada will build a new wastewater treatment plant in 2022 that includes UV disinfection.
* I’ve written before about the challenges of detecting water quality trends. I’m pleased to report that two of the trends discussed here (a reduction in phosphorus at mid-range flow conditions, and a reduction in E. coli in dry and low-flow conditions) were statistically significant at 90% confidence level, using a test of the difference between medians. The approach employed here (sorting by flow conditions) may be a good way to control for weather and reach more reliable conclusions. It’s exciting to have some good news that holds up to further analysis!