Denitrification at a Dairy Site Supported by Gas-Liquid Phase Modeling of Tritium/Helium-3 Groundwater Age
Lawrence Livermore National Laboratory
Surface irrigation incorporating effluent from dairy operations can be a non-point source of nitrate in groundwater. To investigate the source and fate of nitrate at a dairy site in the San Joaquin Valley, California, we determined tritium/helium-3 (3H/3He) groundwater age, nitrate concentration and isotopic composition, and excess nitrogen gas concentration in vertical profiles at several locations in a shallow groundwater system beneath fields irrigated by groundwater mixed with dairy effluent. The nitrogen data provided direct evidence for saturated-zone denitrification in groundwater underlying the site. In this study, we model the transport of 3H and 3He continuously through the unsaturated and saturated zones to determine the source and flux of groundwater in which the denitrification is occurring. Our finding is that the groundwater originates directly from irrigation water applied on the site, which demonstrates that nitrate loading from the dairy operation is being actively mitigated in the local subsurface. Other lines of evidence in conjunction with the nitrogen and groundwater tracer data suggest that the dairy?s nutrient and farm management practices promote subsurface denitrification resulting in remediation of dairy nitrate contamination of underlying groundwater. A novel aspect of this study is that we account for transport of 3H and 3He in the unsaturated zone, where 3He preferentially partitions into the gas phase. By doing so we address a recurring problem in 3H/3He age interpretation - the tritium activity in surface water (generally inferred from IAEA precipitation records) is often assumed for both the source activity for groundwater recharge and the initial activity for the 3H/3He groundwater age dating chronometer. The problem occurs because the 3H concentration history reaching the water table, the location of recharge, can be significantly different than the 3H concentration history in surface water, the location of 3H source measurements. For the dairy site, a conventional 3H/3He age interpretation would suggest tritium activities in recharging groundwater at the water table are higher than seen in precipitation for the calculated recharge year (see figure). Our gas-liquid phase 3H and 3He modeling results show that accounting for travel time and dispersion from transport through the 5-meter thick unsaturated zone to the water table completely resolves this unrealistic discrepancy and confirms the recharge source is irrigation return. In the larger realm of 3H/3He age interpretation, our data and modeling approach show the importance of unsaturated zone transport in reconciling recent controversy sparked by discrepancy between inferred and measured 3H source activity.