Vertical Sampling in Recharge Areas Versus Lateral Sampling in Discharge Areas: Assessing the Agricultural Nitrogen Legacy in Groundwater

Abstract:

Agricultural nitrogen (N) is a legacy contaminant often found in shallow groundwater systems. This legacy has commonly been observed using well nests (vertical sampling) in recharge areas, but may also be observed by sampling at points in/beneath a streambed using pushable probes along transects across a channel (lateral sampling). We compared results from two different streambed point sampling approaches and from wells in the recharge area to assess whether the different approaches give fundamentally different pictures of (1) the magnitude of N contamination, (2) historic trends in N contamination, and (3) the extent to which denitrification attenuates nitrate transport through the surficial aquifer.

Two different arrangements of streambed points (SP) were used to sample groundwater discharging into a coastal plain stream in North Carolina. In July 2012, a 58 m reach was sampled using closely-spaced lateral transects of SP, revealing high average [NO₃^-] (808 µM, n=39). In March 2013, transects of SP were widely distributed through a 2.7 km reach that contained the 58 m reach and suggested overall lower [NO₃^-] (210 µM, n=30), possibly due to variation in land use along the longer study reach. Mean [NO₃^-] from vertical sampling (2 well nests with 3 wells each) was 296 µM. Groundwater apparent ages from SP in the 58 m and 2.7 km reaches suggested lower recharge [NO₃^-] (observed [NO₃^-] plus modeled excess N₂) in 0-10 year-old water (1250 µM and 525 µM, respectively), compared to higher recharge [NO₃^-] from 10-30 years ago (about 1600 µM and 900 µM, respectively). In the wells, [NO₃^-] was highest (835 µM) in groundwater with apparent age of 12-15 years and declined as apparent age increased, a trend that was consistent with SP in the 2.7 km reach. The 58 m reach suggested elevated recharge [NO₃^-] (>1100 µM) over a 50-year period. Excess N₂ from wells suggested that about 62% of nitrate had been removed via denitrification since recharge, versus 51% and 78% from SP in the 58 m and 2.7 km reaches, respectively. Groundwater mean transit times from SP were 26-31 years, similar to preliminary estimates from wells (24 years). The two approaches to streambed point sampling have different strengths and weaknesses compared to vertical sampling in the recharge area, but may be viable alternatives for future assessments of legacy N in groundwater.