Subsurface transport of orthophosphate in five agricultural watersheds, USA
Highlights
► Phosphorus transport in groundwater was assessed at five agricultural watersheds. ► Phosphorus subsurface movement was controlled by sorption onto iron oxides or clays. ► Water chemistry and soil types affected phosphorus transport to the aquifers. ► Low pH conditions within an aquifer in Maryland prevented phosphorus transport. ► Geochemical modeling was used to assess adsorption.
Introduction
Nutrient transport (nitrogen and phosphorus compounds) from agricultural fields to surface and groundwater is one of the most serious environmental problems throughout the world (Salvia-Castellvi et al., 2005, National Research Council (NRC), 2005, Diaz et al., 2004). Eutrophication, or excessive growth of algae, of surface water bodies impairs their use for recreation and domestic consumption and limits their use by native biota. Where surface water is chlorinated and used for drinking water, increased levels of algae contribute to the formation of trihalomethanes and haloacetic acids (Scully et al., 1988; Oliver and Shindler, 1980), which have been shown to be both carcinogenic and mutagenic in laboratory toxicity studies (Boorman, 1999). Recently, Sprague and Lorenz (2009) reported an increase in total phosphorus concentrations in streams in the central United States between 1993 and 2003. The increase in total phosphorus was significantly correlated with an increase in fertilizer use. No significant change in total phosphorus was found in streams in the western or eastern United States.
In natural systems, phosphorus is mainly cycled through a plant-soil dynamic. However in agricultural systems, phosphorus is removed at harvest and must be replenished with either mineral fertilizer or manure. Soils may be over applied with respect to phosphorus because many management programs are tied to nitrogen control or crop needs and phosphorus management is not always considered (Sharpley et al., 2003). This issue is common in areas using manure, which typically has a N:P ratio lower than needed by most crops. Application of manure to meet N requirements often results in the application of P exceeding crop demand. Surface water contamination with phosphorus due to runoff from manured fields (Klausner et al., 1976, Hergert et al., 1981, Allen and Mallarino, 2008) and agriculture in general (Baker et al., 1975, Burwell et al., 1977, Sharpley and Syers, 1979, Vollenweider, 1968) is a well-studied topic.
It is known that phosphorus sorbs to soil particles. Phosphorus sorbs onto oxides of iron and aluminum (Hemwell, 1957, Zhang and Huang, 2007), clay minerals (Parfitt, 1978) and to calcium carbonate (Cole et al., 1953). It has been assumed that phosphorus transport to groundwater is negligible because of sorption and therefore earlier studies on phosphorus transport have been focused solely on surface water pathways (Baker et al., 1975, Burwell et al., 1977, Sharpley and Syers, 1979, Vollenweider, 1968). Attenuation of phosphorus movement and plant availability by interactions onto soil particles has been recognized by previous research (Gunjigake and Wada, 1981, Anderegg and Naylor, 1988, Parfitt, 1978). More recent studies (Devau et al., 2009), for example, have further refined understanding of various factors, such as soil pH, with respect to phosphorus mobility or plant availability and the importance of clay minerals, in addition to iron and aluminum oxides for controlling the distribution of phosphorus between soil and water.
It was hypothesized for this study that phosphorus movement in the unsaturated zone and along a groundwater flow path to a stream or tile drain would be different with location because of hydrological and chemical factors. Processes affecting the rate of water movement through the unsaturated zone are likely to be important for phosphorus transport as well as processes that affect sorption or dissolution. Chemical equilibrium may not be achieved in the unsaturated zone because of short residence times at some locations, if transport times are fast such as might occur because of preferential flow paths, and because of competition for sorption sites from competing ions. It was further hypothesized that groundwater processes affecting phosphorus would more likely be closer to equilibrium relative to the unsaturated zone as residence time increases and groundwater chemistry stabilizes allowing equilibrium conditions between the fluid and the solids. Groundwater/surface water interactions should also differ with location relative to the amount and direction of exchange of water across the streambed interface. Unsaturated zone, groundwater, and streambed materials and water chemistry are expected to affect the transport or immobilization of phosphorus. In a previous study at a set of five watersheds across a range of climatic and soil types within the United States, Domagalski et al. (2008a) showed that baseflow contributed more than 20% of the annual ortho P load in some of the streams of this study. Tesoriero et al. (2009) examined the baseflow index of some of the same streams, as this study, with a range of groundwater contributions to discharge. They found that the baseflow index was negatively correlated with in-stream ortho P concentrations at most sites. However, at sites dominated by baseflow or having favorable geochemical conditions (e.g. aquifers having low dissolved oxygen concentrations or lacking in iron and aluminum oxides, clay minerals, and calcium carbonate) groundwater contributions of ortho P to streams are important (Tesoriero et al., 2009). The purpose of this investigation was to examine the geochemical conditions and reactions affecting ortho P concentrations in the subsurface environmental compartments (unsaturated zone, groundwater, groundwater/surface water interface) of the five watersheds from the Domagalski et al. (2008a) and Tesoriero et al. (2009) studies. Geochemical modeling was used to determine the level of saturation of ortho P with respect to minerals and sorption onto iron oxides. The results of the modeling were used to interpret the subsurface ortho P concentrations and assess the potential for phosphorus transport along the groundwater flow paths.
Section snippets
Study areas
Five study areas were chosen in agricultural watersheds nested within larger basins that are being investigated by the US Geological Survey (USGS) NAWQA Program (US Geological Survey, 2010a) (Fig. 1). Specific characteristics of the aquifers, crop types, and phosphorus use for each study area are shown in Table 1.
Study areas were chosen from a previous investigation that included important agricultural systems and to cover a range of hydrologic settings using the hydrologic landscape concept (
Field methods
Unsaturated-zone monitoring locations were equipped with instrumentation to measure surficial soil temperature, soil heat flux, soil water matric potential and soil moisture at depth in order to estimate the flux of water out of the soil and the amount of water in the soil. Polyvinylchloride lysimeters with ceramic porous cups were installed at various depths to collect unsaturated zone water for chemical analysis by applied suction. The shallowest lysimeter was located just below the root zone
Phosphorus concentrations and water chemistry
Concentrations of ortho P for each hydrological compartment (unsaturated zone, groundwater, groundwater/surface water interface, streams, and tile drains (Indiana site only) are shown in Fig. 2, along with the US EPA criteria for total phosphorus in specific ecoregions (US Environmental Protection Agency, 2010). Concentrations of ortho P in rainfall were below the laboratory-reporting limit of 0.008 mg/L. Therefore rain was not considered a source of phosphorus to the soil.
Concentrations of
Summary and conclusion
Phosphorus can be transported below the root zone to the unsaturated zone and through an aquifer in agricultural settings. This study showed that under conditions where phosphorus is either not entirely taken up by plant tissue or where soil chemistry does not favor either precipitation or sorption, that sub-surface transport can result in elevated concentrations in groundwater or loadings to receiving streams. In this study, phosphorus transport to a stream was demonstrated most clearly at the
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