Research Transport of Phosphate from Soil to Surface Waters by Preferential Flow R E N E G A¨ C H T E R * Swiss Federal Institute of Environmental Science and Technology (EAWAG), Limnological Research Center, CH-6047 Kastanienbaum, Switzerland JOHN M. NGATIAH† International Institute for Infrastructural, Hydraulic and Environmental Engineering, IHE Delft, The Netherlands CHRISTIAN STAMM Institute for Terrestrial Ecology, ETH Zurich, Grabenstrasse 3, CH-8952 Schlieren, Switzerland
Enrichment of lakes with soluble reactive phosphorus (SRP) leads to their deterioration as ecosystems, recreation areas, and drinking water reservoirs. In many cases, fertilized soils are their most important P source. Most studies dealing with P losses from soils to surface waters concentrate on erosion and surface runoff. Leaching is mostly considered to be of minor importance. On the basis of an in-situ sprinkling experiment with dye and bromide as tracers and of observations of the dynamics of SRP concentration and water discharge at the watershed scale, we identify soil macropores and artificial drainage systems as the most important pathways for the vertical and lateral transport of SRP from P-enriched soil surfaces to surface waters. A conceptional model explains why in drainage systems flow rate and SRP concentration are positively related. We estimate that more than half of the yearly SRP load is leached from the soil, and thus conclude that counter to the conventional wisdom, in the investigated watershed, leaching and not surface runoff is the most important mechanism for P transfer from soils to surface waters. It should be tested to see whether this conclusion can be generalized and also hold true for other watersheds with artificially drained, P-enriched soils with a low matrix permeability.
Introduction In most cases, eutrophication of lakes (excessive primary production) leading to their deterioration as ecosystems, impairing their value as recreation areas, and endangering them as drinking water reservoirs is due to elevated phosphorus (P) loads. In many cases, advanced wastewater treatment and the banning of polyphosphates in detergents have significantly lowered P loading. As a consequence, at least on a relative basis, fertilized soils have become increasingly important as a P source. Most studies dealing with P losses from soils to surface waters concentrate on erosion and surface runoff. Leaching * Corresponding author fax: ++41 41 349 21 68; e-mail:
[email protected]. † Present address: Ministry of Environment & Natural Research, P.O. Box 30126, Nairobi, Kenya. S0013-936X(97)00782-7 CCC: $15.00 Published on Web 05/12/1998
1998 American Chemical Society
is mostly considered to be of minor importance (e.g., refs 1-3). This view is supported by the high sorption affinity of dissolved o-phosphate to many soil minerals resulting in strongly elevated P concentrations in the fertilized topsoil but leaving deeper layers of the soil profile nearly unaffected. As a consequence, P concentrations in drainage waters are generally much lower than in surface runoff (4). Nevertheless, some field studies have suggested that subsurface flow may also be important for the P transfer from soils to surface waters (4-14). For the watershed of the river Kleine Aa, this hypothesis is based on the following: (i) Direct evidence obtained from year-round investigations of several drainage systems (13, 14). These measurements consistently yielded increasing SRP concentrations with increasing discharge rates. Furthermore, it was often observed that, soon after the farmers had spread liquid manure, the SRP concentration of affected drains increased drastically and the draining water turned brownish. (ii) Indirect evidence obtained from about 15 000 SRP concentration and discharge measurements carried out during 1 year at a high time resolution (1 sample per 35 min) in the river Kleine Aa (13). This study showed that SRP concentrations peaked consistently after the water discharge peaked and that SRP concentrations were higher on the falling than on the rising limb of the hydrograph. Because surface runoff would be expected to cease shortly after the end of precipitation, the retarded SRP peaks and the lasting elevated SRP concentrations can hardly be explained by surface runoff. However, these observations suggest that seepage was mainly responsible for the elevated SRP concentrations at elevated discharge rates of the river. On the basis of results of an in-situ experiment in combination with the described observations at the watershed scale, we provide in this study a quantitative estimate on the contribution of leaching to the elevated P load of Lake Sempach and identify vertical macropores and lateral drainage systems as important pathways for subsurface P transport. These findings supplement results obtained from other watersheds where saturation of the soil with P has been shown to be mainly responsible for the high P losses to groundwaters and surface waters (8, 15).
Experimental Section Study Site. The river Kleine Aa drains a subcatchment (6.9 km2) at the northeastern part of Lake Sempach situated in the Central Swiss Plateau at an altitude between 505 and 670 m (Figure 1). In the watershed, soils have developed from glacial till (Wu ¨ rm glaciation) and Molasse, and at the experimental site the soil was a loamy Dystric Eutrochrept (16). Average annual precipitation and mean temperature are 1200 mm yr-1 and 7.5 °C, respectively. Seventy-eight percent (5.4 km2) of the subcatchment is used as farmland. Most of the area is cultivated mainly as grassland. Intensive dairy farming and pig production prevail (average livestock density is 3.1 dairy-cow equiv/ha). The grassland is mowed up to seven times during the growing season. Liquid manure is applied as frequently after each cut, resulting in an average application of about 47 kg of P ha-1 yr-1. It is estimated that about 40% of the catchment area is artificially drained and that total losses of soluble molybdate-reactive P (SRP) into the river yield 1.15 kg ha-1 yr-1 (13). River discharge and concentrations of SRP in the river were measured close to its inlet to Lake Sempach (Figure 1) as described by Ga¨chter et al. (13). VOL. 32, NO. 13, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Br- and Dye Accumulation and Soil Dry Weight in the Column Sampled after the Sprinkling Experimenta layer (cm) grass 0-3 3-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 applied to column recovered in column
FIGURE 1. Catchment of the river Kleine Aa (shaded area). Locations of experimental site (E) and stations to sample the river (S) and to measure precipitation (P).
FIGURE 2. Sampling the soil profile following the tracer experiment (according to Ngatiah; 18). Field Experiment. To demonstrate and to evaluate the effect of preferential flow on water and SRP transport, we conducted a sprinkling experiment followed by soil excavation. The tracer experiment had three main goals: (i) to identify pathways for the fast P transport from the soil surface to drainage systems, (ii) to get information about possible retention of P at the walls of postulated macropores, and (iii) to obtain information on the water budget of the soil at the experimental site. A total of 109 L of double-distilled water labeled with sodium bromide (0.12 g L-1) and sodium sulforhodamine (8.33 g L-1) was sprinkled homogeneously on a 1.5 m by 1.5 m plot of grassland during a period of 8 h. The resulting sprinkling intensity of 6 mm h-1 is high but not unusual as compared to naturally occurring precipitation. The day after sprinkling, half of the sprinkled plot was excavated down to the water table (about 1 m below the surface), and 10 layers of soil with an area of 0.5 m by 0.5 m were sampled in the center of the irrigated plot (Figure 2). The uppermost two layers extended from 0 to 3 cm and from 3 to 10 cm. The other eight layers were 10 cm thick. After removing the layers, the resulting horizontal planes at 3, 10, 20, etc. were photographed. The photographs were analyzed for dye coverage. At each plane, soil samples were selectively scraped from the walls or the center of well-defined macropores (dyed spots) and from the unstained area at a distance of 2-2.5 cm from dyed spots. These samples were stored at 4 °C in glass vials closed with screw caps until analysis. Analytical Techniques. To determine the background of Br- concentration, soil samples from the unstained edge 1866
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Br- accumulation (g) (%) 0.00 0.40 0.31 0.11 0.04 0.03 0.00 0.00 0.02 0.02 0.02 1.46
0 27 21 8 3 2 0 0 1 1 1 100
0.95
65
dye content (g) (%)
dry weight of soil (particle size 51%) agree well. Conceptional Model Explaining the Dependence of the SRP Concentration on Discharge Rate in Drainage Systems. According to Ga¨chter et al. (13) and Stamm et al. (14), SRP concentrations observed in drainage pipes were positively related to water discharge. Ga¨chter et al. (13) further observed about equal concentrations in drainage pipes and overland runoff when the latter occurred during an exceptionally heavy rainstorm. Because preferential flow is mainly responsible for the fast transfer of SRP through the soil, the following conceptional model can explain why SRP concentration 1868
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increases with increasing discharge rate: If surface infiltration of precipitated water exceeds its vertical transport across a deeper soil horizon z, then the pore water content will increase above the horizon z, and eventually pore water will start to drain into macropores. The larger the difference is between the water fluxes at the soil surface and at the depth z; and the longer it persist, the higher the water will rise toward the soil surface, and the closer to the soil surface it will drain into macropores. Since the WEP content of the soil increases strongly toward the soil surface (Figure 3), by definition also the SRP concentration of the draining water will increase. If the soil pores are eventually saturated with water up to the soil surface, the SRP concentration of the draining water approaches the concentration of the overland runoff, as observed by Ga¨chter et al. (13). In summary, as rain intensity increases, more water flowing in macropores originates from shallower soil depths. Thus the SRP concentration of the draining water increases as its flow rate increases. Necessary Measures To Lower the Leaching of P from Farmland. Irrespective of the transport pathways bypassing the soil matrix (accelerated vertical flow along macropores or overland runoff), the loss of SRP from soil to surface waters has to be related to the WEP content of the top soil. Thus, in those watersheds where the SRP loss from fertilized, P-enriched farmland is the main cause for the undesired eutrophication of the receiving lake, responsible and sustainable agricultural management must find ways to diminish the WEP stock to the minimum level necessary while still permitting optimum crop production. Although this postulate just seems to agree with common sense, it asks for additional, perhaps costly changes in agricultural practice in those catchments where the farmland is not primarily fertilized to maintain a high crop harvest but to dump the manure originating from a too high livestock fed with imported food. To sum it up, we conclude that vertical macropores in combination with fast lateral water movement (mainly along drainage systems) contribute significantly to the eutrophication of Lake Sempach. During the past 2 decades, preferential flow has been demonstrated to occur in many different soils (19-26). Thus, it seems likely but still needs to be tested whether or not these conclusions are also applicable to other artificially drained watersheds characterized by P-enriched soils with a low matrix permeability.
Acknowledgments We thank H. Behrendt, J. Gelbrecht, W. Giger, T. Gonser, M. Mengis, H. J. Lubberding, J. S. Meyer, J. L. Schnoor, J. Zobrist, and three anonymous referees for their valuable suggestions, which helped considerably clarifying our thoughts and strengthen the manuscript. Figure 1 was drawn by H. Bolliger and L. Zweifel. A. Mares assisted in the laboratory, and J. Leuenberger assisted in the field experiment. The Institute for Terrestrial Ecology provided the sprinkler used for the tracer experiment.
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Received for review September 3, 1997. Revised manuscript received March 30, 1998. Accepted April 7, 1998. ES9707825
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