Bioavailable describes the portion of a substance, such as a nutrient, metal, or contaminant, that is in a form readily accessible for uptake by living organisms, including plants, microorganisms, and aquatic life. In stormwater management and water quality science, bioavailability determines whether a pollutant can actively participate in biological processes and cause ecological effects.
A substance may be present in the environment in many forms, but only some of those forms are bioavailable. For example, nutrients like nitrogen and phosphorus are most bioavailable when dissolved in water as nitrate or orthophosphate, which organisms can easily absorb. In contrast, the same nutrients bound to soil particles or locked in complex organic compounds may be far less accessible and therefore less immediately impactful.
Bioavailability is influenced by several environmental factors, including chemical form, solubility, pH, temperature, and the presence of other substances. For metals, bioavailability often depends on whether they are dissolved as free ions or bound to particles or organic matter. For organic pollutants, it depends on how easily they dissolve in water or can be broken down into usable forms.
In the context of stormwater management, bioavailability is critical because it affects how pollutants impact receiving waters. Bioavailable nutrients can fuel excessive plant and algal growth, leading to problems such as eutrophication and harmful algal blooms. Bioavailable metals and toxic compounds can be directly taken up by aquatic organisms, potentially causing toxicity or bioaccumulation.
Stormwater treatment practices are often designed not only to remove pollutants, but also to reduce their bioavailability. Processes such as adsorption to soil particles, microbial transformation, and chemical binding can convert pollutants into less bioavailable forms, thereby reducing their ecological risk even if they are not completely removed from the system.