Quantifying pollutant loading from channel sources: Watershed-scale application of the River Erosion Model

Abstract

Phosphorus and fine sediment pollution are primary causes of water quality degradation. Streambank erosion is a potentially significant source of fine sediment and particulate phosphorus to watersheds, but it remains difficult to quantify the magnitude of this loading. A new, easily applied, watershed scale model was used to simulate the potential for future phosphorus and sediment loading from channel erosion in two watersheds: Big Dry Creek, Colorado and Lick Creek, North Carolina. The projected magnitude of loading for phosphorus is about an order of magnitude higher in Big Dry Creek compared to Lick Creek (~280 kg/yr and ~50 kg/yr, respectively), while sediment loading results are similar (~950 ton/yr). In both watersheds, model results suggest that channel erosion will not contribute a significant amount of phosphorus to the watershed (~1–4% of historic watershed total from all pollutant sources) but will contribute a large amount of sediment (30–100% of historic watershed total). Uncertainty in these estimates is high, but quantifying confidence in model projections is important for understanding and using model results. Importantly, modeling shows no decrease in loading over the 40-year model time frame in either watershed, suggesting that the channels are not adjusting to a new stable state and erosion will continue to be a pollutant source. Lick Creek model results are sensitive to upstream sediment supply while Big Dry Creek’s are not, reinforcing the importance of considering alterations to both the hydrologic and sediment regimes when analyzing potential channel changes — at least in vertically active channels. This new modeling approach is useful for estimating historic and future phosphorus and sediment loading from channel erosion, an important first step in effective management to improve water quality.

Publication
In Journal of Environmental Management
Date

What we did and why it is important

Nutrient pollution (nitrogen and phosphorus) causes algal blooms - large growths of aquatic plants that deplete oxygen, kill fish, and contaminate drinking water. Nutrient pollution comes from a variety of sources, including wastewater treatment and agricultural and urban runoff. Erosion of stream channels can also contribute significant amounts of pollution, although quantifying this source is difficult. Eroded soil from streambanks is a pollutant on its own, but it can also carry adsorbed phosphorus which may lead to worsening algal blooms. We use a new computer model (the River Erosion Model) to simulate channel erosion and associated pollutant loading in two watersheds - Big Dry Creek, Colorado, and Lick Creek, North Carolina.

These simulations indicate that channel erosion in these two watersheds could be large sources of fine sediment pollution, but rather small sources of phosphorus. In Big Dry Creek, there are other, much larger sources of phosphorus pollution (e.g. wastewater treatment plants) that dwarf the contribution of channel erosion. In Lick Creek, streambank soils have naturally low phosphorus concentrations, so despite the significant channel erosion we simulated, there simply isn’t much phosphorus loading to the stream.

While modeling in our two study areas showed channel erosion wasn’t a large phosphorus source, every watershed is different. The River Erosion Model is a useful new tool to allow anyone to quantify the potential magnitude of pollutant loading in their local streams. And understanding where pollution is coming from is an important first step to stopping it.