4.2.1 What is Water Quality?

Individuals have different opinions about how good surface water quality is defined.  Some focus on water clarity; some on whether there are obvious signs on pollution such as trash or oil sheen; some on the presence or absence of algae.  Most water resource plans focus primarily on the concentration of Total Phosphorus (TP) in lakes, although the MPCA has adopted numeric limits for 126 EPA Clean Water Act priority pollutants.  Very few water bodies are monitored for these priority pollutants because:

  • Most are rarely detected in lakes;
  • When detected, it is even more rare to find concentrations that pose a health risk by ingestion;
  • Even if one is detected at a concentration above standard, it rarely affects use (contact recreation and aesthetics); and
  • One priority pollutant sample scan is about $1000.

For regulatory purposes, the EPA and the MPCA define acceptable water quality as that which supports the designated use of the water resource.  For lakes, those designated uses are recreation and aesthetics; for wetlands it is aquatic life.   Eutrophic conditions are the most common and likely problems impacting use of lakes, and excess nutrients are usually the cause.  The EPA and MPCA regulatory focus is therefore on nutrients, specifically Total Phosphorus (TP) concentration, as a means to classify lakes relative to support of their designated use.   Lakes are determined to be Impaired Waters if their TP concentration exceeds a certain average concentration; in the North Central Hardwood Forest ecoregion in which the District is located, that threshold is 40 ?g/L.

Phosphorus impacts algal and macrophyte productivity, water clarity, fish habitat, aquatic life support, odor, and appearance (aesthetics).  All these factors may be part of an individual lake user’s perception or definition of water quality.  By cost-effective statistical assessments of TP, the “health” of the lake can be measured in terms of nutrient and sediment loads, internal cycling of nutrients, oxygen depletion, macrophyte types and support, aquatic life habitat and aesthetic conditions such as clarity, odor and frequency and types of algal blooms.  In other words, TP is an indicator of water quality as well as a driver of water quality. 

This Plan assumes that good water quality is achieved when the physical, chemical, biological and aesthetic characteristics of a waterbody support its full designated use (recreation, aesthetics and/or aquatic life) and when the ecological integrity of the environment is supported.   Because water quality in lakes is regulated mainly by the TP concentration, the water quality focus of the Plan is on reducing phosphorus loads to the lakes to achieve regulatory TP standards.  However, each subwatershed plan sets forth an integrated set of goals, policies, and actions intended to address other aspects of water quality such as aquatic vegetation, buffer management, biological management, water clarity, and public information and education.