THE INFLUENCE OF AGRICULTURE ON THE UPPER GRADAŠČICA CATCHMENT RIVER WATER QUALITY (SLOVENIA)

Our study attempts to quantify temporal and spatial components of the influence of agricultural practices on the river water quality in a small catchment in central Slovenia. The upper reaches of the Gradaščica river (from the source to the edge of Ljubljana) have been chosen because the catchment is small and primarily rural. Of 442 active farming households, 332 specialize in animal husbandry. Cumulative temporal variability of river water quality is monitored using Hydrolab Miniprobe 14 days 4 times/year (spring, summer, fall and winter). Probe measures 11 parameters every 30 minutes: temperature, specific conductivity, pH, air pressure, dissolved oxygen (mg/l , % sat.), redox potential, depth (0-25 m), salinity, and the concentrations of ammonium and nitrates. Occasionaly, upstream water quality is measured at 6 points using RQflex rapid tests for nitrate, ammonium, and phosphate. Water is sampled for laboratory verification of pH, NO3- and NH4+. Agricultural loading of nutrients will be estimated using available spatial data (digital relief, geology, soil data, information on present agricultural practices) using GIS tools. Preliminary results of measurements from spring 2006 show strong diurnal and weather related trends in most of measured parameters. Temperature and pH peak during daytime; dissolved oxygen, redox potential, salinity, total dissolved solids and nitrates during the night. Rainy episode (53 mm over 5 days) accompanied with cooler temperatures increased the levels of nitrate and ammonium in river water from 1 to 3.5 mg/l N-NO3-, and from 0.5 to 0.15 mg/l N-NH4+, but the values did not exceed warning values for surface waters. Ammonium concentrations returned to background values the next day, wheras nitrate concentrations remained somewhat eleveated (2-2.5 mg/l N-NO3-). Upstream measurements of NO3- and NH4+ failed to detect any spatial trends so far (measured only at low water levels). Phosphates were not detected. The measurements will continue until spring 2007.