Groundwater nutrient concentrations near an incised midwestern stream: effects of floodplain lithology and land management

Abstract

It has been recognized that subsurface lithology plays an important role in controlling nutrient cycling and transport in riparian zones. In Iowa and adjacent states, the majority of alluvium preserved in small and moderate sized valleys consists of Holocene-age organic-rich, and Wne-grained loam. In this paper, we describe and evaluate spatial and temporal patterns of lithology and groundwater nutrient concentrations at a riparian well transect across Walnut Creek at the Neal Smith National Wildlife Refuge in Jasper County, Iowa. Land treatment on one side of the stream reduced the grass cover to bare ground and allowed assessment of the eVects of land management on nutrient concentrations. Results indicated that groundwater in Holocene alluvium is very nutrient rich with background concentrations of nitrogen, phosphorus and dissolved organic carbon that exceed many environmentally sensitive criteria. Average concentrations of ammonium exceeded 1 mg/l in several wells under grass cover whereas nitrate concentrations exceeded 20 mg/l in wells under bare ground. Phosphate concentrations ranged from 0.1 to 1.3 mg/l and DOC concentrations exceeded 5 mg/l in many wells. DenitriWcation, channel incision, land management and geologic age of alluvium were found to contribute to variable nutrient loading patterns at the site. Study results indicated that riparian zones of incised streams downcutting through nutrient-rich Holocene alluvium can potentially be a signiWcant source of nutrient loadings to streams.

Agricultural conversion of floodplain ecosystems: Implications for groundwater quality

“…Nitrate concentrations increased from <2 mg/l to more than 11 mg/l at a toeslope landscape position following land use change to row crops (Zhou et al, 2010). During a riparian zone restoration, Schilling and Jacobson (2008) observed nitrate concentrations increasing from <1 to 40 mg/l when the overlying grass cover was removed.…”

“…Other sources of NO 3 eN to groundwater at the crop site can be ruled out as major contributing factors to the increase. Prior to planting, the grass was burned off and the soil cover was left bare, possibly leaving the site vulnerable to mineralization losses (Schilling and Jacobson, 2008). Zhou et al (2010) reported an increase of approximately 10 mg/l in groundwater following brome grass conversion to row crops at a toeslope landscape position.…”

“…Similarly, at a floodplain site in south-central Iowa, reed canary grass was removed from the floodplain for a restoration project (burning and glyphosate treatment) and the ground was left bare for much of a year. No fertilizer additions were made yet groundwater NO 3 eN concentrations increased from less 1 mg/l to 40 mg/l due to N mineralization and water leaching through bare floodplain soils (Schilling and Jacobson, 2008). The difference in effects was likely due to the amount of time allowed for mineralization to occur.…”

Water and nutrient discharge to a high‐value terrace–floodplain fen: resilience and risk

“…Co-occurrence of NH 4 -N and NO 3 -N in fen groundwater and DO concentrations above 1 mg/l imply that denitrification may be occurring primarily in small microenvironments, where DO has been depleted and reducing conditions are maintained (Jacinthe et al, 1998;Schilling and Jacobson, 2008). Palta et al (2014) recently reported that denitrification was optimized at a site where conditions exhibited both anaerobic and aerobic conditions, indicating that pore-scale hydrology is a major controller of wetland denitrification.…”

Evaluation of analytical and numerical approaches for the estimation of groundwater travel time distribution

“…Channel incision has significantly impacted the watershed hydrology (Schilling and Jacobson, 2008) and is associated with the stream's flashy discharge in response to storm events. An artificial subsurface drainage network at the site is used to drain the water table up to depths of around 1.2 m below the land surface.…”