- Key pollutants of concern have been identified Nitrogen, phosphorus, sediment and pathogens have been identified as the most important pollutants to address with this plan. These pollutants pose risks to human health and the environment, which are outlined within this chapter in greater detail. Strategies to prevent increases in stormwater rates and volumes also need to be considered, as stormwater tile flow and runoff are the largest carriers of these pollutants to the receiving streams. Local flooding has also caused damage to private property and infrastructure.
- Water quality monitoring data is valuable Data collected by the Iowa Soybean Association / Agriculture's Clean Water Alliance (ISA/CWA) and the IOWATER volunteer monitoring program has been valuable to identify pollutant loads and their potential sources.
- Nutrient levels appear higher in rural areas Monitoring data has demonstrated that levels of nitrogen and phosphorus compounds are usually seen at higher levels in the rural areas within this watershed.
- Bacteria levels appear higher in urban areas Observed levels of E.coli bacteria have been much higher within the urban landscape, although levels all across the watershed were consistently above the state’s water quality standard.
- Key sources of sediment loading Streambank erosion, construction sites and gully erosion are projected to be the leading current sources of sediment loading to Walnut Creek. Almost 30,000 tons of sediment per year are estimated to be delivered from the Walnut Creek watershed to the Raccoon River.
- Small footprint, big impact Construction sites make up only 0.1% of the watershed on average each year but contribute significantly to the overall sediment load.
HOW DO THESE CONCEPTS INFLUENCE DEVELOPMENT OF THE PLAN? To develop targeted, effective solutions, the key pollutants posing the greatest risk need to be identified. The likely sources of these pollutants need to be identified so that effective practices can be implemented to achieve the desired load reductions.
Pollutants of Concern
Key pollutants of concern within the Walnut Creek watershed have been defined by considering the following information gathered through development of the watershed plan:
- Review of past studies, including the Water Quality Improvement Plan for the Raccoon River (TMDL), the Raccoon River Water Quality Master Plan and Iowa’s Nutrient Reduction Strategy.
- Review of past local and municipal watershed assessments and storm water infrastructure studies.
- Collection of stakeholder input at WMA meetings, open houses and individual conversations.
- An overview of the available water quality monitoring information collected from sites within the watershed.
Nutrients (Nitrogen and Phosphorus)
Nutrients like nitrogen and phosphorus exist in both surface and ground water under natural conditions. Their presence supports the growth of algae and aquatic plants, providing food and habitat for fish and other aquatic life within streams and lakes. Excessive algal growth can occur when the levels of these nutrients are too high. Algal blooms can block sunlight below the surface, clog fish gills, reduce habitat quality and diminish habitat. The death and decay of algae can lead to diminished oxygen levels, known as hypoxia. Oxygen levels can fall to a range where fish and other wildlife may be sickened or killed. As of 2013, there were 166 of these hypoxic “dead zones” that had been identified in and around the United States. The largest of these areas was a 5,840 square mile area (approximately 10% of the size of the entire state of Iowa) in the Gulf of Mexico, largely attributed to nutrient polluted runoff received from the Mississippi River watershed (which includes the entire state of Iowa).
Excessive algal growth can also increase growth of bacteria and other human pathogens. In some cases, algae can form toxins which can cause rashes, stomach/ liver illness, respiratory and neurological effects in humans. Direct exposure to this algae affects fish and other wildlife, with the toxic impacts being carried up the food chain if they are consumed by other animals. If toxic algae enter into the water supply stream, they can be converted into dioxins through the use of chemical disinfectants in the water treatment process. Risks to human health from dioxins include cancers, reproductive and developmental issues.
(1) Nitrate has been observed at elevated levels in stream flows within the Walnut Creek watershed. Des Moines Water Works' main intake point for surface water from the Raccoon River is located just downstream of where Walnut Creek flows into that river. Nitrates have been known to cause illness and death of human infants when at high levels.
Pathogens are the most common cause for water quality impairment in the United States, with nearly 11,000 waterbodies listed as impaired for this cause in 2014. Pathogens are microscopic organisms which can cause disease in humans or animals. These include viruses, bacteria, protozoa and parasitic worms. The likely presence of pathogens is typically identified by measuring levels of fecal indicator bacteria (FIB) such as Escherichia coli (E. coli) or fecal coliform. Elevated levels of these indicator species demonstrate that conditions are favorable for pathogens at a level which could impact human health when exposures occur.
The primary concern is incidental human ingestion during recreational contact, resulting in illness. In addition, respiratory, skin, ear and eye infections are also possible. Those most at risk are the very young, those with compromised immune systems and those with no prior exposure to the pathogen. The level of exposure required to cause illness varies with each type of pathogen.
A certain amount of sediment is naturally present and transported in streams. However, the excessive loadings observed within this watershed can have significant impacts on water quality and stream structure.
High sediment loads directly impact watershed ecology through habitat loss, reduced wetland functions and impaired water quality in ponds and lakes. Sediment impacts the physical characteristics of waterbodies through decreased floodplain volumes (increases flood risk), higher stream velocities, accelerated streambank erosion, and reduced storage in ponds and lakes. Water quality is also directly affected, as some pollutants are able to bind to sediment particles and be carried downstream.
Runoff Rates and Volume
Stormwater runoff caused by rain, snowmelt and groundwater movement are the main ways that pollutants are carried from the landscape to receiving waters. As volumes of surface and subsurface runoff increase, a larger load of nutrients, pathogens and sediments are likely to be driven to the stream. Understanding the activities that increase the rates and flows of runoff can help us identify potential sources of such increases to address.
Activities that reduce the soil’s ability to soak up water lead to increases in stormwater runoff volume. In rural areas, compaction can occur when large equipment is driven over or through the soil during agricultural activities. Tilling, fertilizing, harvesting and tile installations are all activities which can compact soils. In urban areas, soil compaction primarily occurs by use of heavy equipment during grading and construction operations as part of land development. Installation of impervious surfaces such as roofs, driveways, parking lots and streets can virtually eliminate infiltration which increases runoff volume.
Runoff volume is also increased when native plants, trees and other vegetation are removed. Their root structures dig deep into the soil, helping to keep it loose and porous. They provide habitat for worms, insects and burrowing animals which also increase void spaces within the soil.
As landscapes are developed, many changes also effect the speed at which runoff is funneled to streams. In agricultural areas, many ditches and tiles were constructed to drain wetlands and low lying areas. In some areas, the alignments of larger streams were straightened. Roads were installed with ditches and culverts. In urban areas, impervious areas collect runoff and quickly funnel it into gutters and storm sewers. The pipe network very quickly routes this runoff to the nearest pond or stream.
The combination of these effects results in a system which has been significantly altered from natural conditions. By modeling a case study of a developing area, the effects of these changes can be seen. Runoff volumes in both the agricultural watershed and suburban environments are likely to be several times higher than pre-settlement conditions.
In rural areas, peak rates of flow may be nearly 20 times higher than pre-settlement levels during the most frequently occurring storms. In the suburban environment, peak rates for these events are expected to be 20-45 times more than the natural conditions. These drastic changes demonstrate how storm events of less than three inches of rain can cause rapid rises in stream levels and flash flooding. These effects are likely most dramatic in smaller streams in urban developed areas. These quick bounces account for a significant amount of streambank erosion on an annual basis, leaving the streambanks weakened and vulnerable for more significant impacts from the more rarely occurring larger events.