PHYSICOCHEMICAL PATTERNS WITHIN A HYPORHEIC ZONE OF A NORTHERN MICHIGAN RIVER, WITH COMMENTS ON SURFACE-WATER PATTERNS

Interstitial temperature and chemistry were examined longitudinally and with depth in a hyporheic zinc beneath a riffle-pool sequence of a third-order, sand-bottom river in northern Michigan (USA). Longitudinal and depth patterns were compared with surface and groundwater chemistries at the site and with surface chemistry over a 10-km length of the river. Interstitial water was more characteristic of surface water at the upstream end of the hyporheic zone and of groundwater at the downstream end. Hyporheic longitudinal and depth patterns occurred for temperature, chloride, silica, soluble reactive phosphorus, and winter dissolved oxygen. Dissolved organic carbon decreased with depth, but not with longitudinal distance downstream within the hyporheic zone. Hyporheic nitrate and ammonium patterns were variable, but concentrations generally increased downstream. Hyporheic specific conductance and alkalinity decreased with distance downstream, but not with depth. Gradients for temperature, chloride, silica, nitrate, and phosphate over the 10-km surface water reach were similar to those observed within the single hyporheic zone. Hyporheic chemistry patterns were consistent with models of underflow through porous substrata implying groundwater/surface water interaction. Importance of the hyporheic zone should not be overlooked in solute transport or stream nutrient budgets.