Predicting Nutrient and Contaminant Transport Via Fractures in Glacially-Related Fine-Grained Soils Using Field and Laboratory Soil Texture Data.

Fractures (i.e., secondary porosity via macropores) in glacially related fine-grained soils and parent materials create preferential flow paths for surface-to-agricultural drainage tile transport and ground water aquifer recharge, allowing nutrients and other pollutants to infiltrate rapidly. However, it can be difficult to detect fractures on site without natural exposures or test pit excavations and soil borings. A practical predictive model was developed based on soil texture data from 145 sites in Ohio, 98 sites in Wisconsin, and a few additional sites in Michigan and Iowa, plus results from 32 laboratory scale fracturing experiments. In the laboratory experiments, samples of soils found to be naturally fractured in the field were mixed with increasing proportions of pure silica sand and desiccated to determine at what point the mixtures would no longer support fracturing. Results were plotted on a USDA ternary diagram to define the boundaries of the fracture-prone region of soil textures. The results showed that glacially related fine-grained soil materials having less than 79% sand or greater than 6.5% clay are more likely to support fracturing. Soil scientists can apply this boundary condition method to their own site-specific data to predict where fracturing is most likely to occur in their location and thus where agricultural drainage tile water and ground water will be most vulnerable to nutrient and pollutant transport.  This screening process can also be used to determine which soils, if agriculturally drained, would most benefit from the installations of agricultural tile bioreactors modified to remove both dissolved reactive phosphorus (DRP) and dissolved nitrates which fuel Harmful Algal Blooms (HAB) in Ohio lakes and reservoirs.