The Thornapple River Watershed (TRW) is the largest subbasin of the Lower Grand River Watershed, and is located in the southwestern portion of Lower Michigan. Draining approximately 422,545 acres, the TRW planning area includes 24 subwatersheds and nine designated trout streams. In assessing current watershed conditions, it was discovered that the most critical issues in the TRW are the least visible. Well-water testing indicates that in certain areas, groundwater, which provides the only source of drinking water throughout the watershed, is contaminated with high levels of nitrate, with levels exceeding state water quality standards in approximately 30% of wells in these contaminated areas. Sources of contamination include historic and current agricultural practices, sewage systems and lawn fertilization Click to read the full Thornapple River Watershed Management Plan.