EVALUATING THE CONTRIBUTIONS OF SEDIMENT AND ADJACENT WETLAND PORE WATERS TO THE DISSOLVED ORTHOPHOSPHATE LOAD OF THE KALAMAZOO RIVER

Our goal was to quantify the contributions of water traveling through either riverbed sediment or adjacent wetlands to the PO₄^-3 load of the North Branch of the Kalamazoo River. The 10-km study reach is a meandering and predominantly gaining stream with abundant springs and adjacent wetlands and is upstream from any continuously discharging wastewater treatment plants. Gradient ranges from 1 to 6 ft/mile; most reaches are close to the average of 2.5 feet/mile. The dominant land use is agriculture.

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.