SOLUTE TRANSPORT AND FILTERING THROUGH A RIPARIAN FOREST

A two and a half year study was conducted to determine the fate of sol utes applied to the landscape position just above the upper edge of a forested riparian zone. Nitrate (NO3-N) and bromide (Br) sources were applied to an 18 x 18 m killed grass sod area at the rates of 188 and 336 kg ha(-1) N and Br, respectively. Soil samples were collected four times during the study to determine vertical and lateral transport wi thin the treatment area. Shallow groundwater wells were installed at t ransect positions 13 and 6 m upslope, and 2, 7, 12, 17, and 22 m downs lope from the grass sod-riparian forest interface, There were two sets of wells at each transect position in the treatment area, and three s ets of wells at each transect position in the forested riparian zone, The well depths for each set were 0.5, 1.0, 1.5, and 2.0 m. Water samp les were collected biweekly except for the final six months of the stu dy, when they were collected monthly. Water samples were analyzed for NO3-N and Br content. Mass balance calculations based on soil sample a nalyses showed that the equivalent of 59 and 78% of the applied NO3-N and Br had left the treatment area after one year. High NO3-N concentr ations observed in well waters in the upslope end of the treatment are a showed that shallow groundwater entering from upslope was carrying N O3-N in addition to that applied. Comparisons of Br and NO3-N concentr ations in the wells and also Br/NO3-N ratios showed that the riparian forest was effectively filtering NO3-N entering from the treatment are a. Very little NO3-N was observed in wells 7 and 12 m into the forest. Bromide concentrations observed in the wells were relatively high at 2, 7, and 12 m into the forest. Mean Br/NO3-N ratios showed that some Br had reached as far downslope as 22 m into the riparian forest by th e end of the second year of the study. One pulse of Br appeared in the riparian forest 8 to 11 months after application and a second pulse w ith greater concentrations appeared 16 to 18 months after application. The first pulse is believed to be due to transport via macropore flow , while the second may have been associated with Br initially retained in the soil matrix. Overall the study showed that in areas of the Coa stal Plain where shallow lateral water movement is a dominant loss pat hway, agrichemicals applied in the spring move downslope during the we t months, and depending on soil type, may take several seasons before completely leaving the upslope area and moving into or through the rip arian forest.