a method of phosphorous reduction in stormwater runoff using iron humate , such as in the form of a filter , a layered filter bed , a stacked wall or a liner . the stormwater runoff is sent to a fluid retention area such as a retention pond , wetland reservoir or the like where the runoff is filtered through iron humate . while the runoff is filtered , the iron humate absorbs or chemically retains the phosphorous in the runoff to produce filtered runoff with a reduced level of phosphorous . in an alternate embodiment , the runoff can be pumped from a retention pond into a iron humate filter where the runoff is filtered . in another alternate embodiment , the iron humate filter may be placed in a trench below ground to intercept and filter groundwater flows .