Water Quality


Trophic State Index


Trophic State Indices (TSIs) are an attempt to provide a single quantitative index for the purpose of classifying and ranking lakes, most often from the standpoint of assessing water quality. In recent years the Carlson (1977) Index appears to have attained general acceptance in the limnological community as a reasonable approach to this problem.

TSI ranges along a scale from 0-100 that is based upon relationships between secchi depth and surface water concentrations of algal chlorophyll, and total phosphorus for a set of North American lakes. Its major assumptions is that suspended particulate material in the water controls secchi depth and that algal biomass is the major source of particulates.

The lowest value of zero would correspond to a secchi depth of 64 meters (greater even than Crater Lake, Oregon and Lake Tahoe, CA/NV at its clearest back in the 1960's)! A value of 100 would correspond to a secchi of only 6.4 cm (less than 3 inches- yuck !). A set of equations were then derived to describe these relationships with higher values corresponding to increased fertility, that is, more eutrophic. An increase in TSI of 10 units corresponds to a halving of secchi depth and a doubling of phosphorus concentration.


 Sources of Contaminants

  Surface water can become contaminated in many ways. Surface water can be contaminated when hazardous substances are discharged directly from an outfall pipe or channel or when they receive contaminated storm water runoff. Direct discharges can come from industrial sources or from certain older sewer systems that overflow during wet weather.

Storm water runoff becomes contaminated when rain water comes into contact with contaminated soil and either dissolves the contamination or carries contaminated soil particles. Surface water can also be contaminated when contaminated groundwater reaches the surface through a spring, or when contaminants in the air are deposited on the surface water. Contaminated soil particles carried by storm water runoff or contaminants from the air can sink to the bottom of a surface water body, mix with the sediment, and remain.

Minimum Impact Design Standards


"Minimal Impact Design Standards (MIDS) represent the next generation of stormwater management in Minnesota. The emphasis today is on keeping the raindrop where it falls in order to minimize stormwater runoff and pollution and preserve natural resources. Low Impact Development (LID) is an approach to stormwater management that mimics a site’s natural hydrology as the landscape is developed and preserves and protects environmentally-sensitive site features such as riparian buffers, wetlands, steep slopes, valuable (mature) trees, floodplains, woodlands and highly permeable soils. The MIDS project offers guidelines, recommendations, and tools that will help implement LID more uniformly across Minnesota's landscape and it provides guidance to effectively implement the concepts and practices that LID promotes and encourages. MIDS contains four main elements to meet these needs:

                                                             -Minnesota Stormwater Manual

For a link on how to download the MIDS Calculator to help with designing a stormwater plan for a project site, click here: 

MIDS Calculator

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