WATER-QUALITY STRATIFICATION IN SHALLOW WASTE-WATER STABILIZATION PONDS

The physical limnology of three modern wastewater stabilization ponds serving a small community in Minnesota was investigated over a 1-year period (July 1989 - October 1990). Water temperatures and associated m eteorological parameters were recorded continuously; underwater light, dissolved oxygen, pH, and Secchi depth were measured intermittently ( about weekly). Measurements of nutrients and planktonic species were m ade by other investigators. Water quality stratification dynamics were studied by analyzing variations of water temperature, dissolved oxyge n, and pH distributions with time and over depth. Intermittent stratif ication and mixing of the shallow waste stabilization ponds (1 - 2 m d eep) were documented and related to weather. The strong response of th e ponds to seasonal and daily weather variations was observed. Three t ypes of pond stratification conditions have been identified: (1) compl etely mixed during consecutive day and night, (2) stratified during th e day and well-mixed during the night, and (3) continuously stratified during day and night. A diurnal cycle of stratification dynamics was first noticed in late April and persisted through summer and into fall . Differences in light attenuation and hence temperature stratificatio n and DO distribution between pond 1 (primary), pond 2 (second primary ) and pond 3 (secondary) in the wastewater treatment system were docum ented and related to different waste loading conditions. Temperature s tratification affects chemical, microbial, and planktonic processes in the ponds. Results presented in this paper can be used to provide gui dance for water quality sampling in monitoring of pond performance. In formation on true mixing conditions is also needed to gain better unde rstanding of important factors affecting pond operation, and for proce ss simulations and reader modeling of waste stabilization ponds.