Environ. Sci. Technol. 2010, 44, 908–914
Impact of Point-Source Pollution on Phosphorus and Nitrogen Cycling in Stream-Bed Sediments E L I Z A B E T H J . P A L M E R - F E L G A T E , * ,† ROBERT J. G. MORTIMER,‡ MICHAEL D. KROM,‡ AND HELEN P. JARVIE† Centre for Ecology and Hydrology, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire, OX10 8BB, United Kingdom and School of Earth and Environment, University of Leeds, LS2 9JT, United Kingdom
Received September 7, 2009. Revised manuscript received December 16, 2009. Accepted December 17, 2009.
Diffusive equilibration in thin films was used to study the cycling of phosphorus and nitrogen at the sediment-water interface in situ and with minimal disturbance to redox conditions. Soluble reactive phosphate (SRP), nitrate, nitrite, ammonium, sulfate, iron, and manganese profiles were measured in a rural stream, 12 m upstream, adjacent to, and 8 m downstream of a septic tank discharge. Sewage fungus adjacent to the discharge resulted in anoxic conditions directly above the sediment. SRP and ammonium increased with depth through the fungus layer to environmentally significant concentrations (440 and 1800 µM, respectively) due to release at the sediment surface. This compared to only 0.8 µM of SRP and 2.0 µM of ammonium in the water column upstream of the discharge. Concomitant removal of ammonium, nitrite and nitrate within 0.5 cm below the fungus-water interface provided evidence for anaerobic ammonium oxidation (anammox). “Hotspots” of porewater SRP (up to 350 µM) at the downstream site demonstrated potential in-stream storage of the elevated P concentrations from the effluent. These results provide direct in situ evidence of phosphorus and nitrogen release from river-bed sediments under anoxic conditions created by sewage-fungus, and highlight the wider importance of redox conditions and rural point sources on in-stream nutrient cycling.
Introduction Nutrient enrichment of lowland rivers is of global concern. Stream cycling of both P and N can affect the nutrient supply available for primary production, as well as for export downstream. River and stream bed-sediments have been shown to play a key role in the cycling of P and N in rivers (1, 2). They have also been shown to act as important sinks for bioavailable P and N (3, 4), but may switch to a source due to chemical, biological, or physical changes within the stream (5, 6). It is therefore important to understand processes controlling pore water nutrient concentrations in streambed sediments. Work in freshwater systems has shown the importance of redox conditions on the uptake and release of P and N (7, 8). * Corresponding author e-mail:
[email protected]. † Centre for Ecology and Hydrology, Maclean Building, Crowmarsh Gifford. ‡ School of Earth and Environment, University of Leeds. 908
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 3, 2010
Although studies have been undertaken in riverine systems, much of this work has been based on laboratory mesocosm experiments, e.g., refs 9 and 10. Due to the sensitivity of these sediment processes to redox controls and in situ environmental conditions, this is not ideal. DET (diffusive equilibration in thin films) technology allows the steep chemical gradients at and below the sediment-water interface to be measured (11). Crucially it is an in situ technique with short equilibration times that involves minimal disruption to the sediment and redox profiles. DET and its sister technology, DGT (diffusive gradients in thin films), have been widely used to study metals and nutrients in marine environments (12, 13). Recently, the technique has been applied to measure pore water P profiles and fluxes in rivers (14, 15). However, to date there has been no published work using DET probes to study pore water N profiles and cycling at the sediment-water interface in rivers. Here, we use DET probes to measure sediment pore-water profiles of soluble reactive phosphate (SRP), nitrate, nitrite, ammonium, sulfate, iron, and manganese for a lowland headwater tributary subject to septic tank discharge. These are used to examine the impact of redox conditions and rural point-sources on potential P and N release from stream bed sediment. To the authors’ knowledge, this is the first in situ study of both P and N and associated redox chemistry in stream bed sediment at the resolution required to determine the nature of the biogeochemical processes occurring.
Experimental Section Study Site. Lone Pine Pasture stream is a headwater tributary of the Eye brook, which drains into the River Welland in Leicestershire (eastern England). It is on intensively underdrained, steeply sloping soils developed over Lias clay with ironstone. The stream drains a 1.2 km2 catchment, predominantly composed of grassland supporting sheep farming, as well as some arable land. The site receives an effluent discharge from septic tanks in Loddington village, which serve a residential population of ∼30, and is representative of many rural streams in Britain. The discharge also receives field and road runoff during rainfall events. These headwater streams often have a relatively low baseflow discharge, meaning effluent is diluted less than inputs further down in the catchment. Three sampling sites were selected: 12 m upstream of the discharge (“upstream”), adjacent to the discharge (“adjacent”), and 8 m downstream of the discharge (“downstream”). Reach Characterization. Two sampling surveys were undertaken, one in the spring (April 2007) and one in the summer (July 2007). In-situ oxygen profiles of the water column and sediment were taken at submm resolution using Unisense microsensors (April survey only). Flow was measured at the upstream and downstream site at the time of sampling using an acoustic Doppler velocity meter. A streamwater sample was taken from the upstream, adjacent, effluent, and downstream sites at the time of probe retrieval. A surface (
