Major Findings > Water Quality Characterization

Water Quality in Drainage Ditches When Flow is Dominated by Agricultural Tile Drainage

Water samples were taken approximately once per month at three sites on each of the six drainage ditches (shown on the linked detailed map). All stations were sampled as nearly at the same time as possible; a typical sampling run took about two hours. Samples were delivered to the Heidelberg College Water Quality Laboratory, where they were analyzed for suspended sediment, total phosphorus, soluble reactive phosphorus, nitrate, nitrite, ammonia, total Kjeldahl nitrogen, dissolved reactive silica, specific conductance, chloride, sulfate, and fluoride.

Results

As of June 15, 2002, 192 samples have been analyzed, with initial sampling in May 2001. These samples include two sets of samples from high flow conditions (October and December 2001), three sets from conditions of low to no flow (July, August, and September 2001), and seven sets of samples from conditions of moderate flow dominated by tile discharge.

Concentrations observed in these samples were generally comparable to those observed in larger rivers in NW Ohio (Maumee, Sandusky), though concentrations of suspended solids were lower than typical for these rivers.

Perhaps the most important finding is that, when flows are dominated by tile discharge, nitrogen/phosphorus ratios (total N/total P) are much higher than would be ideal for biological assimilation of these nutrients. While a ratio between 10 and 20 would be desirable, these ratios are more typically in the range of 60 to nearly 800. This suggests that nutrient uptake will be phosphorus-limited, and much of the nitrogen will not be taken up by the aquatic ecosystem. Unless denitrification is an active process at these times, substantial nitrogen export is to be expected.

When flows are dominated by tile discharge, concentrations are similar from station to station, and greater differences are seen from month to month than from station to station. All stations tend to change in the same way from month to month. In other words, the stations tend to show homogeneous behavior.

During summer low flow conditions, station-to-station differences become more substantial, and concentrations change differently at different stations from month to month. The homogeneous behavior that characterized periods of tile flow is not apparent during low flow. Concentrations of some nutrient species can be extremely high, probably reflecting the influence of septic effluent in combination with minimal dilution in the stream. Concentrations of organic nitrogen in excess of 4 grams/liter, and total phosphorus in excess of 1 gram/liter were observed in one sample, reflecting extremely high algal densities. These concentrations are more than 1000 times higher than normal. Nitrate, ammonia, and soluble phosphorus tend to be low at these times, because they are measured as dissolved constituents, and dissolved nutrients are largely incorporated into algae and other plant biomass.

Suspended sediment concentrations were generally rather low. The highest values were recorded in August 2001, but algae dominated these samples: the "suspended sediment" was biological in nature, not mineralogical, as is typically the case.

One station, Rader Creek at County Line Road, shows evidence of being impacted by a local point source of pollutants. It often behaves differently than the other stations from month to month, and frequently has the highest concentrations of nitrate, total phosphorus, and soluble reactive phosphorus.

Elevated concentrations of total and soluble phosphorus were observed in October 2001 at two stations on Needles Creek and two stations on Bull Creek. The reason for these anomalous concentrations is unknown.