The Intensive Survey Unit (ISU) collects and interprets a variety of biological, chemical, and physical data that are incorporated in the DWQ basinwide planning concept. Numerous special studies are conducted including lake watershed assessments, water quality characterization studies for model support, sediment evaluations for oxygen demand, nutrient flux, and chemical contamination, and a variety of more intensive water quality investigations. Water quality simulation models are often used for the purpose of constructing wasteload allocations so that appropriate wastewater limits can be included as requirements in National Pollutant Discharge Elimination System (NPDES) permits. Where large financial expenditures or the protection of water quality is at risk, models should be calibrated and verified with actual in-stream field data. Because sufficient historical data are often lacking, intensive water quality surveys are required to provide the field data necessary to accomplish model calibration and verification. Intensive water quality surveys are performed on water bodies below existing and proposed wastewater discharges and usually consist of time-of-travel dye study, flow measurement, physical and chemical samples, long-term biochemical oxygen demand (BODlt) analysis, water body channel geometry, and/or effluent characterization analysis. If oxygen depletion is suspected due to the characteristics of benthic sediments then sediment oxygen demand (SOD) studies may be performed.

Lakes are valued for the multiple benefits they provide to the public, including recreational boating, fishing, drinking water, and aesthetic enjoyment. The ISU seeks to protect these waters through monitoring, pollution prevention and control, and restoration, and public education activities. Assessments have been made at many publicly accessible lakes, at lakes which supply domestic drinking water, and lakes (public or private) where water quality problems have been observed. Data are used to determine the trophic state of each lake, a relative measure of nutrient enrichment and productivity, and whether the designated uses of the lake have been threatened or impaired by pollution.