Nitrate Dynamics in Relation to Lithology and Hydrologic Flow Path in a River Riparian Zone Kevin J. Devito,* Dan Fitzgerald, Alan R. Hill, and Ramon Aravena ABSTRACT The efficiency with which riparian zones remove nitrate (NO~-) from contaminated ground water can vary with landscape setting. This study was conducted to determine the influence of flood plain geometry, lithology, hydrologic flow path, and nitrate transport on mechanisms of nitrate depletion of contaminated groundwater. Pat- terns of NOj--N, chloride, and dissolved organic carbon (DOC) con- centrations and ~1~N-NOj~ and ~lsO-NO~- values in combination with detailed piezometric head measurements were investigated in a river floodplain connectedto a large uplandsand aquifer in an agricultural region near Alliston, Ontario, Canada. Ground water discharging to the forested floodplain fromthe sand aquifer exhibited large spatial variability in NO~--N concentrations (10-50 mg/L). The transport and depletion of NO~- was strongly influenced by floodplain geometry and lithology. Little ground water flow occurred through the low- conductivity matrix of peat in the floodplain. Plumesof NOj--rich ground water passed beneath the riparian wetland peat and flowed laterally in a 2- to 4-m-thick zone of permeable sands across the floodplain to the fiver. Analyses of the distribution of the NO~--N concentrations, isotopes, and DOC within the floodplain indicate that denitriflcation occurred within the sand aquifer near the fiver where nitrate-rich ground water interacted with buried channel sediments andsurface water recharged from peat to the deeper sands. This study shows that the depth of permeableriparian sediments, groundwater flow path, and the location of organic-rich subsurface deposits may be moreimportant than the width of vegetated strips in influencing the ability of riparian zones to remove nitrate. N ITRATE contamination of ground water from agricul- tural practices is well-documented (Spalding and Exner, 1993; Hamilton and Helsel, 1995) and has raised concerns about surface water quality throughout North America and Europe. Riparian zones (vegetated strips and wetlands located between streams or lakes and up- lands) represent critical interfaces within a landscape. Dueto their location, these buffer zones have the poten- tial to regulate energy and material fluxes between up- lands and surface waters (Gregory et al., 1991; Hill, 1996; Cirmo and McDonnell, 1997). Riparian wetlands have been promoted as a means of reducing NOj- in ground water contaminated from agriculture and other humanactivities (Peterjohn and Correll, 1984; Muscuttet al., 1993; Haycock et al., 1993; Gilliam, 1994). However, reviews by Hill (1996) Cirmo and McDonnell (1997) illustrate the complexity of hydrologic and biogeochemical interactions in the near-stream zone. Despite extensive research, the role K.J. Devito, Dep. of Biological Sciences, Univ. of Alberta, Edmonton, AB Canada T6G 2E9. D. Fitzgerald and R. Aravena, Dep. of Earth Sciences, Univ. of Waterloo, Waterloo, ON Canada N2L 3G1. A.R. Hill, Dep. of Geography, York Univ., Toronto, ON Canada M3J 1P3. Received 22 Mar. 1999. *Corresponding author (kevin.devito@ ualberta.ca). Published in J. Environ. Qual. 29:1075-1084 (2000). of riparian zones in nitrate removal from ground water is still unclear. Much of the data suggesting that riparian zones effec- tively remove NO~- have been obtained from sites where small ground water inputs are restricted by an aquitard at depths of 1 to 3 mto shallow lateral subsurface flow paths that maximize the interaction with riparian soils and vegetation (e.g., Peterjohn and Correll, 1984; Coo- per, 1990; Haycock and Pinay, 1993; see Hill, 1996). Less effective NO~-retention has been suggested by research on riparian areas connected to large upland aquifers with more vertical direction of flow through sediments, or areas that have internal hydrologic flow paths dominated by surface transport (Hill, 1990; Mc- Dowell et al., 1992; Devito and Dillon, 1993; Phillips et al., 1993; Hedin et al., 1998). However,manyof these studies have been conducted on sites located in forested landscapes with low NO~- inputs. Few studies have ex- amined the NO;- dynamics of riparian zones along head- water streams or larger rivers that receive high ground water NO;- inputs from thick extensive aquifers in ag- ricultural landscapes. In these landscapes, the subsur- face permeability of thicker riparian sediments and the greater depth or absence of an aquitard are important factors that influence the flowpath and removal of NO~--contaminated ground water. Most previous stud- ies, with the exception of a recent study by Cey et al. (1999), have used a few transects consisting of single pipes with large (0.5 to >1 m) slot zones that encompass only 1 to 2 m of the near-surface substrate (Hill, 1996). Consequently, there is a lack of data on the three-dimen- sional complexity of hydrology and chemistry interac- tions, which are particularly important in riparian zones connected to thick aquifers. Thus, more detailed studies relating NO~- dynamics to subsurface hydrology within riparian zones from a broader range of hydrogeologic settings are required to develop conceptual models of riparian zone nutrient dynamics and to generalize their potential to retain NO~- throughout agricultural land- scapes. The reduction of NO~- concentrations in ground water as it passes through the riparian zone has been attributed to plant uptake (Jordan et al., 1993; O’Neil and Gordon, 1994), denitrification by microbes (Groffman et al., 1992; Pinay ct al., 1993), or dilution (Hill, 1990; B6hlke and Denver, 1995; Komorand Magner, 1996). There is considerable uncertainty about the relative importance of these mechanisms of NO;- removal and their relation- ship to ground water flow patterns. In addition, most studies have focused on NO;- removal in the surface soil or adjacent to the water table (Cooper, 1990; Low- rance, 1992; Pinay et al., 1993). However, recent studies Abbreviations: DOC, dissolved organic carbon. 1075 Published July, 2000