Date

Fall 2021

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)

Department

Moss Landing Marine Laboratories

Abstract

Shallow groundwater and shallow groundwater nitrogen have been suspected to influence agricultural tile drains, agricultural drainage ditches, and estuaries within the Lower Salinas Valley (LSV) of California’s Central Coast. This study used geochemical tracers to evaluate the influence of groundwater to each of these water sources. For agricultural sites, groundwater discharge estimates revealed between 51% ± 16% to 95% ± 30% of tile drain water was sourced from shallow groundwater. Stable isotopes of water (𝛿2HH2O and 𝛿18OH2O) confirmed that sump-influenced ditches are influenced by tile drain discharge, and that tile drains are influenced by shallow groundwater. Further, average nitrate as nitrogen (NO3-N) concentrations revealed that NO3-N in sumpinfluenced ditches were an order of magnitude higher (i.e., 33.78 to 95.21 mg L-1 NO3-N) than non-sump-influenced drainage ditches (i.e., 3.38 to 8.50 mg L-1 NO3-N). Nitrogen concentrations of shallow groundwater were also significantly lower than those of tile drain and sump water, which suggested that shallow groundwater was not the main source of nitrogen to agricultural drainage water. Stable isotopes of nitrate (𝛿15NNO3 and 𝛿18ONO3) within sump-influenced ditches were similar to those in tile drain effluent. However, groundwater nutrient discharge estimates revealed that 2.9 ± 0.7 to 5.4 ± 1.2 kg/d NO3-N of the total 9.4 ± 2.1 kg/d NO3-N from tile drains comes from shallow groundwater, further suggesting that legacy nutrients in shallow groundwater were not the primary source of nutrients to tile drains. Finally, statistical analyses (ANOVA and PERMANOVA) of nitrogen tracers reveals a lack of seasonality in agricultural drainage system nutrient content that requires further investigation to evaluate correlation with annual NO3-N variability of local estuaries and waterways (e.g., Moro Cojo Slough). This study is the first assessment of shallow groundwater influence to agricultural drainage systems via tile drains in the LSV and provides essential information for regional growers regarding nutrient water quality monitoring and best management practices, particularly in light of recent regulatory adoption of the Irrigated Lands Regulatory Program (Ag Order 4.0).

Geochemical tracers were also employed to evaluate the influence of shallow groundwater on characteristic wet season NO3-N increases observed within California Central Coast estuaries. During February 2019, the characteristic NO3-N spike was observed in Moro Cojo Slough, the Moss Landing Harbor, Monterey Bay, Elkhorn Slough, and the Old Salinas River. NO3-N concentrations decreased in Moro Cojo Slough during the dry season, which highlighted the annual variability of nutrients associated with Central Coast estuaries. Radon-222 (222Rn) activities in Moro Cojo Slough surface water did not increase between wet season or dry season downstream monitoring. However, activities were greater along the channel length during the 2019 wet season (2.58 ± 1.39 dpm L-1 222Rn) than during the 2019 dry season (0.81 ± 0.57 dpm L-1 222Rn). Using surface water and groundwater 222Rn activities, groundwater discharge estimates revealed that advective groundwater flux remained low during both seasons in Moro Cojo Slough. Shallow groundwater nitrogen flux estimates revealed that groundwater was not a major source of nitrogen to Moro Cojo Slough during the wet season. Elevated dry season shallow groundwater NH4-N concentrations suggested that groundwater may significantly contribute to dry season surface water nitrogen in Moro Cojo. 222Rn activities in Elkhorn Slough (2.38 ± 1.42 dpm L-1 222Rn) were similar in magnitude to Moro Cojo Slough, while 222Rn activities in the Old Salinas River were an order of magnitude higher (25.0 ± 4.25 dpm L-1 222Rn ). Paired with our findings from Old Salinas River watershed agricultural drainage ditches and tile drains, we argue that elevated 222Rn activities in the Old Salinas River were from 222Rn-rich tile drain discharge rather than from advection of shallow groundwater to the channel. These findings highlight that groundwater via advective flux is not a significant source of water or nitrogen to California Central Coast estuaries, but that shallow groundwater discharge via tile drains plays an important role within the watershed.

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