[Source: Keith Schilling, Iowa Geological Survey at the University of Iowa, 07.2016 | Keywords: Water, Water Quality, Iowa]
Although it took several decades for significant progress to be seen on the land, practices such as contour cropping, terracing and grass waterways were adopted, slowing runoff and reducing soil erosion and sediment export.
Media outlets and the public quickly pounce on the latest news of a spill or water quality violation, believing that this event is proof that water quality is getting worse. But trends are a tricky business. If you say that water quality is getting worse, this implies that it has changed over time. Do you mean water quality conditions are worse compared to last year, 10 years ago, 100 years ago or since pre-settlement? It is important to consider the changes in water quality that have occurred in Iowa over the last 150 years.
It would be safe to say that Iowa’s water quality was best before Euro-American settlement when Iowa consisted of tallgrass prairie and savanna. We do not have any monitoring information from this time but nutrient levels in streams were likely very low, streams were connected to their floodplains, and the landscape was dominated by infiltration rather than runoff.
Compared to this time period, conditions today are unmistakably worse. However, even in this pristine system, it must be understood that floods still happened, streams still eroded their banks (30 percent of banks in natural meandering streams are severely eroding), and nitrogen was moving through the ecosystem.
It did not take long following settlement that water quality conditions took a turn for the worse. Over the next 50 years, early settlers turned over the prairie, exposed nutrient rich soils to mineralization, plowed up and down slopes with moldboard plows, drained wetlands and tiled wet areas, straightened streams, and generally transformed the landscape to one dominated by runoff processes.
As Ding Darling’s cartoons captured, soil erosion in the early 20th Century was a severe and massive problem. In response to more runoff, streams rapidly incised and widened into their floodplains. It was also during this time period that cities were rapidly expanding and contributing to stream degradation. Reports from the 1920s indicate farmers were complaining that streams were open sewers from cities and that city wastes were impacting livestock. Overall, this was not a good time for water quality. It could be argued that water quality in the early 20th Century was considerably worse than today, so the trend since the 1920s represents tremendous improvement.
In the decades that followed, from the 1930s to 1970s, there were many water quality improvements. Cities built wastewater treatment plants and eliminated raw sewage being dumped into rivers. Passage of the Clean Water Act in 1972 mandated significant reductions in many discharge pollutants such as ammonia-nitrogen. During the Dust Bowl years, it was recognized that soil conservation practices were needed and the Soil Conservation Service was founded. Although it took several decades for significant progress to be seen on the land, practices such as contour cropping, terraces, and grass waterways were adopted that slowed runoff and reduced soil erosion and sediment export.
Evidence for reduced sediment export is found in the Raccoon River in west-central Iowa. Our research has shown that sediment export was significantly reduced in the watershed despite an increase in river flow and the number of tilled row crop acres. Although the largest annual sediment load exported from the Raccoon River occurred in 1972 when row crop acres rapidly expanded during the Earl Butz “freedom to farm” years, the general trend in sediment export since the 1930s has been decidedly downward.
On the other hand, nitrate levels in our rivers are a different story. Limited water quality data from 1906 indicated that nitrate concentrations in several major rivers were less than 1 mg/l and we can imagine that pre-settlement concentrations were even lower. We see evidence for this at the Neal Smith National Wildlife Refuge where my research has shown nitrate concentrations draining restored prairie areas are essentially zero.
Although prairie soil is very organic rich, it does not leach nitrate because the continuous grass cover sequesters excess water and nutrients throughout a long growing season. Farming with tillage and annual crops breaks the continuous cycle and allows the water to leach out nitrate during the spring and fall when there is no crop growth. A convergence of agricultural management changes occurring in mid-20th Century, including increased mechanization (removing much of the demand for small grains and perennial rotations) and increasing commercial fertilizer usage, contributed to increasing nitrate levels in Iowa rivers. Average concentrations were 3-4 mg/l in the 1950s and increased to 7-8 mg/l in rivers today. During spring, concentrations routinely exceed 10 mg/l in many rivers.
The influence of row crop farming on stream nitrate concentrations is profound — our research has shown you can estimate the mean annual nitrate concentrations in Iowa rivers by simply multiplying the percentage of cropped land in its watershed by 0.1. This means that watersheds with 80 percent row crop
can expect to see average annual nitrate concentrations of about 8 mg/l. So trends in nitrate concentrations over the 20th Century are definitely upward.
Finally, what are water quality trends over the last few decades? Despite an increase in monitoring activity, results are pretty inconclusive. Levels seem to rise and fall with climate variability, higher during wet months or years and lower during dry periods. Care must be taken to decipher trends in water quality over the last decade as weather dominates year-to-year changes. The same monitoring record can show both increasing and decreasing trends depending on the starting and stopping points in the time series.
We recently analyzed nitrate concentrations in 46 Iowa streams from 1998 to 2013 and found that 80 percent of the rivers showed no significant change. Those rivers that did show a change were increasing, and interestingly they were all located in western and southern Iowa. Overall, given the changes in land use patterns and urban and agricultural practices that occurred over the last 100-plus years, water quality conditions today are generally stable relative to the past. This can be considered both good news and bad news.
The good news is the worst is probably behind us and there are stable benchmarks to judge future progress toward water quality improvements. The bad news is that given the huge investments society made in conservation and urban infrastructure to stabilize water quality over the course of the 20th Century, it will probably take a similar investment to realize our 21st Century water quality goals.
Keith Schilling is a research scientist with the Iowa Geological Survey at the University of Iowa.
Source: In Clay, R. and Runyan, C. (eds.) Getting Into Soil and Water 2016, Iowa Water Center, Ames, IA