Source terms and pathways. Those are what analysts talk about when explaining how contaminants move in the environment. Plant nutrients nitrogen (N) and phosphorus (P) can be flushed out of the soil, or leached from decaying plant residues on diffuse land-sources, and transported downhill into the lake in water flowing along pathways that are the network of ditches, streams, tributaries, and stormwater systems. Around the lake shore, subsurface septic field seepage can add to the nutrient load.
Source terms are all around us as this map illustrates. Agricultural land used for growing annual or perennial crops and deciduous and coniferous forests are the largest diffuse sources of nutrients. Urbanized land, including the Town of Sylvan Lake and five Summer Villages, are also important sources of nutrients. Even protective riparian zones can release nutrients from decaying organic plant residues. Point sources, from which spilled contaminants can migrate into stormwater or streams, need strict safety practices to avoid releases into the lake.
Soils generally like to hold on to nutrients in both their pore space water films and on the surfaces of minerals and organics that make up the soil mass. There are several soil types in the watershed so the availability of nutrients varies with location, cropping history, forest and shrub cover, fertilizer application, land contours., local hydrology, urban landscape and other factors. Use the Alberta Agriculture Soil Viewer for more detail.
Colored land areas delineate the variety of soils inside the red-line perimeter of the watershed. Click to enlarge the image.
Water flows off the land downhill into the lake in a network of roadside ditches and tributaries that drain several precipitation catchments that are defined by the topography. Municipal roads function as dams or berms to intercept and divert cross-county surface flow through the next culvert. The yellow contour lines in the following map define the most likely flow paths for precipitation or snow melt surface water
Municipal road ditches (in white) and tributaries (in purple) are the channels that drain water from the land into the lake. The Town of Sylvan Lake captures much of its stormwater and directs it eastward out of the watershed to the Red Deer River. Click to enlarge the image.
The release of nutrients into water bodies in Alberta has been studied extensively by Alberta Agriculture. The following important series of reports on Alberta Environmentally Sustainable Agriculture explains how N and P can transfer in Spring snow melt and later in April-October precipitation runoff:
- AESA Summary and Recommendations-Volume 1
- AESA Soil Quality Monitoring Project-Volume 2
- AESA Water Quality Monitoring Project-Volume 3
- AESA Nitrogen Loss in Surface Runoff-Volume 4
- AESA Predicting Phosphorus Losses from Agricultural Areas-Volume 5
When precipitation saturates the top few centimeters of soil, soluble minerals, plant nutrients, and other organics and decay products can diffuse and release into mobile water and flow by gravity into collection streams. If precipitation and surface flow is rapid then soil erosion can release and suspend fine soil particles and other debris that leads to solids transport downstream. See a Town of Sylvan Lake case history here. Alberta’s Water Act states that contamination of a water body is illegal.
Depending on the mineral type, fine soil particles like clays with high surface areas can adsorb and retain nutrients and then deposit them as sediment when discharged into Sylvan Lake. Phosphate species readily stick to fine solid particles and help to keep them suspended in water. That’s how phosphate detergents work. Because of the pioneering Experimental Lakes research of Prof. David Schindler phosphate detergents are regulated substances as major water bodies were becoming eutrophic when fertilized by high concentrations of the nutrient phosphorus from domestic sources.