EVALUATING THE CONTRIBUTIONS OF SEDIMENT AND ADJACENT WETLAND PORE WATERS TO THE DISSOLVED ORTHOPHOSPHATE LOAD OF THE KALAMAZOO RIVER
Seepage rates (cm/sec) assessed at 6 locations range from 0.79E-5 to 7.7 E-5 and average 3.2E-5. Average PO4-3 concentrations (mg/l) range from 0.11 to 0.5 and average 0.3 (n=20). This is significantly higher than the river concentration of .038 (n=66). There is a positive correlation between PO4-3 and dissolved iron in seeped water, suggesting reductive dissolution of iron hydroxide contributes to the elevated PO4-3. By gauging and sampling the river along a 4.5 km reach, we calculated the increase in Q and loads of NO3-, PO4-3, SO4-2. Applying our seepage loads to the area of riverbed within the reach, we calculate that seepage contributes about 80% of the PO4-3 and 20% of the Q, but only small amounts of the NO3- and SO4-2. Previous work has shown springs carry high NO3-and SO4-2. A best-fit mass balance mixing model between spring and seep water predicts river water with similar PO4-3, SO4-2 and seep:spring ratio, but grossly over predicts the NO3-.
The wetlands are within 1m of the river’s bank-full stage and have a layer of peat 1m thick over silty sand similar to river sediment. We sampled 10 wells with depths from 1m to 1.5m. PO4-3 concentrations (average 0.08 mg/l n=24) were highly variable. Reasoning that during the falling limb of the storm hydrograph we would see water draining wetlands, we measured the concentration of ions and monitored stage during storm events. Following storms, load of all ions increased. Because of uncertainty due to low overall PO4-3concentrations and calculating discharge from stage, we feel this needs more work.
The most significant contributor to PO4-3 in the river is seepage from the riverbed sediment, evidenced by the high concentration of PO4-3 in the water collected by seepage meters. The wetlands contribution is difficult to quantify, but presently appears less significant.