Daily and Seasonal Changes in the Water Quality of Irrigation Containment Ponds

We have been continuously monitoring water quality parameters in containment ponds at five commercial nursery and greenhouse operations in Maryland and Virginia, since 2011.  Diurnal changes in pH, EC, water temperature, dissolved oxygen (DO), oxidation-reduction potential (ORP), chlorophyll AB, and blue-green algae counts were measured every 15 minutes.  Environmental conditions (rainfall, air temperature and relative humidity, photosynthetically-active radiation [PAR] and wind speed) were continuously measured every 5 minutes at all locations.  Results from all locations showed surprisingly similar seasonal patterns, where water temperature and EC tended to increase from early spring, peak in early Fall and then decrease to a minimum in late winter (averaging 0.20 to 0.5 dS∙m^-1).  Water temperatures, EC, and turbidity were typically stratified with depth and were affected by rainfall events.  Longer-term pH dynamics were damped by influxes of water from runoff events, and appeared to be limited primarily by minimum water temperatures and PAR on a seasonal basis. When temperature and PAR limitations were removed, diurnal changes in pH were rapid and extreme, ranging from pH 6.8 to 10.5, often within a few hours. Rapid changes in pH appear to be tightly coupled with DO concentrations, peaking at 4:00–5:00 PM and returning to minimum values between 6:00–7:00 AM.  Changes in pH therefore appear to be driven by the daily photosynthetic activity of microorganisms in the water column, especially in ponds with good water quality, where changes in pH are poorly buffered.  Despite relatively low EC levels, it appears that none of these ponds are nitrogen or phosphorus limited, which appears to be the primary catalyst for algal productivity.