Preferential Groundwater Flow in Riparian Wetlands: Effects on Nutrient Delivery J. Angier1 and G.W. McCarty1 1 USDA-Agricultural Research Service, Beltsville, Maryland 20705 INTRODUCTION: Vegetated riparian corridors (buffer strips) are often considered as natural remediation sites for agricultural contaminants, capable of removing excess nutrients before they enter rivers and streams. In the case of nutrient-rich groundwater, a reducing soil zone should perform this function, provided the buffer strip is of a certain minimum width (usually 100 feet). The ideal groundwater flow model includes horizontal flow through a riparian soil zone, with sufficient contact time between the groundwater and the soil matrix to allow nutrient removal to occur, prior to discharge directly into the stream. However, significant deviation from this model can minimize the effectiveness of nutrient removal. Preferential flow pathways can be an important influence on the hydrology, and thus on the remediation capacity, of a site. Heterogeneity within the soil structure, and extensive macroporosity, allow significant amounts of contaminated groundwater to reach the surface relatively rapidly, effectively bypassing natural remediation conditions. A fairly small portion of the total soil may conduct most of the groundwater that ends up being discharged. If much of the contaminated groundwater is discharged to the surface (and exposed to oxic conditions) before reaching the stream, the remediation capacity of the riparian zone is further diminished. Total width of a riparian buffer strip is therefore not the best predictor for riparian zone function, particularly in the presence of preferential flow pathways, which are common in these environments. IDEALIZED RIPARIAN FLOW SYSTEM NITRATE FLUX IN STREAM: Nitrate concentrations in stream vary from <1mg/L to >9mg/L. The greatest increase in nitrate flux is between stations 2&3. During baseflow, nitrate flux increases between stations 2&3, but decreases downstream During high flow, nitrate flux increases at each station downstream. 25-65% of total increase in nitrate flux between stations 2&3 is supplied by one secondary channel input to the stream. HYDRAULIC GRADIENTS, UPWELLING ZONES, AND SECONDARY CHANNELS: Within the floodplain of the riparian zone, positive vertical hydraulic gradients are significantly greater than horizontal gradients. Permanently saturated surface areas (upwelling zones) appear where vertical hydraulic gradients are particularly high. Secondary channels that run parallel to the stream often originate in or near these upwelling zones. These secondary channels can carry nearly as much water as the main stream channel during baseflow conditions. SCALING PREFERENTIAL FLOW: Preferential flow pathways can operate on a range of scales. Macropores typically function on a relatively small (centimeter) scale, determined by pore diameter and length. Even in extensive interconnected macropore systems, the scale of an individual flow path is limited by the degree of connectivity between pores; that is, the length of a specific continuous unobstructed pathway. Larger areas may also function as discreet preferential flow pathways. Continuous layers of high-hydraulic conductivity (K) material in the subsurface can act as preferential flow paths in their ability to transmit more groundwater than the average soil matrix. Such extensive layers of high-K substrate have been mapped at this site, with continuous sand lenses on the meter+ scale. These conductive layers coincide with perennially upwelling surface areas, indicating that their presence has an effect on groundwater flow. While the delivery of groundwater to the surface in these zones may be facilitated by the presence of a macropore system, the sand layers present a larger area over which preferential flow can occur. It is likely that interaction between macropores and high-K layers enhances preferential flow in this system; high conductivity layers within the soil allow groundwater to move rapidly within the soil, while macropores enable contaminated groundwater to emerge from the soil zone onto the surface. When exfiltration rates exceed the water usage capacity (ET) of the upwelling area, the emergent groundwater is channelized across the surface and into the stream. Vertical hydraulic gradients over a one year period for nested piezometer transect BP, which intersects major groundwater upwelling zone in floodplain. Nest BP5, which is in a highly active exfiltration zone, is always strongly positive. BP4, closer to the hillslope, varies according to season and precipitation conditions; note the dramatic reversal in gradient in response to flooding caused by hurricane Floyd (mid-late September, 1999). Horizontal gradients from the field to the stream are very weakly positive throughout. DISCHARGE ADDED PER UNIT STREAM LENGTH Note that the largest increase in stream flow is nearly always between stations 2 & 3. About 40% of that increase is supplied by a single dominant secondary channel system. PHOTO OF "MINIWEIR" IN SECONDARY CHANNEL SIDE-VIEW (PROFILE) WITH DISSOLVED OXYGEN VALUES Transect across riparian zone, intersecting major secondary channels. Inverted triangles indicate screened interval for each piezometer in nest. Average vertical hydraulic gradients listed in italics above each nest. Note that for nests in highly active upwelling zones (evidenced by perennial surface saturation and consistently high vertical gradients), dissolved oxygen from the underlying (oxic) aquifer penetrates up through the (largely anoxic) soil, indicating significant (and rapid) movement of groundwater up through the soil profile. Where gradients are weak, there is little or no upward penetration of oxic groundwater. SIDE-VIEW (PROFILE) WITH NITRATE CONCENTRATION VALUES Nitrate concentrations listed for each piezometer, and at surface. Once again, there is significant penetration of nitrate up through the profile where gradients are strong, little or no upward movement of nitrate in inactive areas. SOIL CORE Soil core extracted from active upwelling area. Note the prominent sand layers at approximately 80, 110, and 130 cm below the surface. These layers are found at similar depths throughout the area where large upwelling zones and high nitrate surface water concentrations are also found. Piezometers intersecting these layers typically have K values at least an order of magnitude greater than surrounding layers. The "post-settlement" layer, containing a greater mineral component (indicating the change in land use to agriculture in this region) is also clearly visible. The rest of the riparian soil profile averages about 25% organic matter (OM). CONCLUSIONS: Preferential flowpaths can dominate the subsurface hydrology of the soil. Upwelling zones and secondary channels can supply significant amounts of contaminated water to a stream. Horizontal flow through the riparian zone may not be the principal direction of groundwater flow. Deviations from idealized flow can significantly diminish the remediation capacity of a riparian zone.
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