Technical note: An open-source, low-cost system for continuous monitoring of low nitrate concentrations in soil and open water

Nitrate (NO3-), mainly leaching with soil porewater, is the primary nonpoint source pollutant of groundwater worldwide. Obtaining real-time information on nitrate levels in soils would allow for gaining a better understanding of the sources and transport dynamics of nitrate through the unsaturated zone. However, conventional nitrate detection techniques (e.g., soil sample analysis) necessitate costly, laboratory-grade equipment for analysis, along with human resources, resulting in a laborious and time-intensive procedure. These drawbacks raise the need to develop cost-effective and automated systems for in situ nitrate measurements in field conditions. This study presents the development of a low-cost, portable, automated system for field measurements of nitrate in soil porewater and open water bodies. The system is based on the spectrophotometric determination of nitrate using a single reagent. The system design and processing software are openly accessible, including a building guide, to allow duplicating or changing the system according to user-specific needs. Three field tests, conducted over 5 weeks, validated the system's measurement capabilities within the range of 0–10 ppm NO3-–N with a low RMSE of &lt;0.2 ppm NO3-–N when comparing the results to standard laboratory nitrate analysis. Data derived from such a system allow for tracking of the temporal variation in soil nitrate, thus opening new possibilities for diverse soil and nutrient management studies.</p

Pipes to Earth's subsurface: the role of atmospheric conditions in controlling air transport through boreholes and shafts

Understanding air exchange dynamics between underground cavities (e.g., caves, mines, boreholes, etc.) and the atmosphere is significant for the exploration of gas transport across the Earth–atmosphere interface. Here, we investigated the role of atmospheric conditions in controlling air transport inside boreholes using in situ field measurements. Three geometries were explored: (1) a narrow and deep shaft (0.1&thinsp;m wide and 27&thinsp;m deep), ending in a large underground cavity; (2) the same shaft after the pipe was lowered and separated from the cavity; and (3) a deep large-diameter borehole (59&thinsp;m deep and 3.4&thinsp;m wide). Absolute humidity was found to be a reliable proxy for distinguishing between atmospheric and cavity air masses (mainly during the winter and spring seasons) and thus to explore air transport through the three geometries. Airflow directions in the first two narrow-diameter geometries were found to be driven by changes in barometric pressure, whereas airflow in the large-diameter geometry was correlated primarily with the diurnal cycles of ambient atmospheric temperature. CO2 concentrations of  ∼ 2000&thinsp;ppm were found in all three geometries, indicating that airflow from the Earth's subsurface into the atmosphere may also be significant in the investigation of greenhouse gas emissions.</p

Identifying Agricultural Managed Aquifer Recharge Locations to Benefit Drinking Water Supply in Rural Communities

The southern Central Valley of California is one of the most productive agricultural regions in the world. Yet, decades of groundwater use beyond sustainable yield have left rural communities highly vulnerable to shortages and contamination of their drinking water supply. As state regulation begins to address these issues, a need exists to design adaptive and appropriate management systems to increase resilience of rural communities. Targeted managed aquifer recharge on agricultural land (Ag-MAR) near rural communities is one such strategy that could potentially stabilize groundwater tables and maintain or improve groundwater quality in domestic supply wells. Here we present a geographic information system-based multicriteria decision analysis that combines biophysical data (soils, land use, and surface water conveyance) with groundwater modeling and particle tracking to identify suitable agricultural land parcels for multibenefit groundwater recharge within well capture zones of 288 rural communities. Parcels are prioritized using a vulnerability index to change in groundwater supply, derived from well reliance and failures, pesticide applications, land subsidence, and socio-economic data. Our analysis identifies 2,998 suitable land parcels for Ag-MAR within the well capture zones of 149 of the 288 communities, of which 144 rely mainly on groundwater for drinking water. The majority of identified Ag-MAR parcels serve communities ranked as having extreme or very high vulnerability to changes in groundwater supply. Our research produces new understanding of factors contributing to community vulnerability and resilience to changes in drinking water supply and can be used to discuss actions to help achieve a stable and high-quality water supply

Agricultural managed aquifer recharge (Ag-MAR)—a method for sustainable groundwater management: A review

More than two billion people and 40% of global agricultural production depend upon unsustainable groundwater extraction. Managed aquifer recharge (MAR), the practice of strategically recharging water to replenish subsurface storage, is an important subbasin scale practice for managing groundwater more sustainably. However, it is not yet reaching its full potential to counterbalance growing global groundwater demand. Agricultural managed aquifer recharge (Ag-MAR) is an emerging method for spreading large volume flows on agricultural lands and has capacity for widespread global implementation. Yet, knowledge gaps, synergies, and tradeoffs in Ag-MAR research still exist. We identify six key system considerations when implementing Ag-MAR: water source, soil and unsaturated zone processes, impact on groundwater, crop system suitability, climate change and impact on greenhouse gas emissions, and social and economic feasibility. We describe the present distribution, need for common terminology, and benefits of Ag-MAR including groundwater storage, increased environmental flows, and domestic wells support. We then outline major gaps, namely, water quality impacts, and crop health and yield. We showcase the multidisciplinary approach needed for communication and coordination of Ag-MAR programs with stakeholders and the public and provide a framework for implementation. Finally, we outline a vision for the path to Ag-MAR implementation. Ag-MAR is an important approach for achieving groundwater sustainability. However, it is one of many necessary solutions and does not offset the need for groundwater conservation

An underground, wireless, open-source, low-cost system for monitoring oxygen, temperature, and soil moisture

The use of wireless sensor networks to measure soil parameters eliminates the need to remove sensors for field operations, such as tillage, thus allowing long-term measurements without multiple disturbances to soil structure. Wireless sensors also reduce above-ground cables and the risk of undesired equipment damage and potential data loss. However, implementing wireless sensor networks in field studies usually requires advanced and costly engineering knowledge. This study presents a new underground, wireless, open-source, low-cost system for monitoring soil oxygen, temperature, and soil moisture. The process of system design, assembly, programming, deployment, and power management is presented. The system can be left underground for several years without the need to change the battery. Emphasis was given on modularity so that it can be easily duplicated or changed if needed and deployed without previous engineering knowledge. Data from this type of system have a wide range of applications, including precision agriculture and high-resolution modelling

Molecular and Dual-Isotopic Profiling of the Microbial Controls on Nitrogen Leaching in Agricultural Soils under Managed Aquifer Recharge

Nitrate (NO3-) leaching is a serious health and ecological concern in global agroecosystems, particularly those under the application of agricultural-managed aquifer recharge (Ag-MAR); however, there is an absence of information on microbial controls affecting NO3- leaching outcomes. We combine natural dual isotopes of NO3- (15N/14N and 18O/16O) with metagenomics, quantitative polymerase chain reaction (PCR), and a threshold indicator taxa analysis (TITAN) to investigate the activities, taxon profiles, and environmental controls of soil microbiome associated with NO3- leaching at different depths from Californian vineyards under Ag-MAR application. The isotopic signatures demonstrated a significant priming effect (P &lt; 0.01) of Ag-MAR on denitrification activities in the topsoil (0-10 cm), with a 12-25-fold increase of 15N-NO3- and 18O-NO3- after the first 24 h of flooding, followed by a sharp decrease in the enrichment of both isotopes with ∼80% decline in denitrification activities thereafter. In contrast, deeper soils (60-100 cm) showed minimal or no denitrification activities over the course of Ag-MAR application, thus resulting in 10-20-fold of residual NO3- being leached. Metagenomic profiling and laboratory microcosm demonstrated that both nitrifying and denitrifying groups, responsible for controlling NO3- leaching, decreased in abundance and potential activity rates with soil depth. TITAN suggested that Nitrosocosmicus and Bradyrhizobium, as the major nitrifier and denitrifier, had the highest and lowest tipping points with regard to the NO3- changes (P &lt; 0.05), respectively. Overall, our study provides new insight into specific depth limitations of microbial controls on soil NO3- leaching in agroecosystems

Nitrogen fate during agricultural managed aquifer recharge: Linking plant response, hydrologic, and geochemical processes.

Agricultural managed aquifer recharge (Ag-MAR, on-farm recharge), where farmland is flooded with excess surface water to intentionally recharge groundwater, has received increasing attention by policy makers and researchers in recent years. However, there remain concerns about the potential for Ag-MAR to exacerbate nitrate (NO3-) contamination of groundwater, and additional risks, such as greenhouse gas emissions and crop tolerance to prolonged flooding. Here, we conducted a large-scale, replicated winter groundwater recharge experiment to quantify the effect of Ag-MAR on soil N biogeochemical transformations, potential NO3- leaching to groundwater, soil physico-chemical conditions, and crop yield. The field experiment was conducted in two grapevine vineyards in the Central Valley of California, which were each flooded for 2 weeks and 4 weeks, respectively, with 1.31 and 1.32 m3 m-2 of water. Hydrologic, geochemical, and microbial results indicate that NO3- leaching from the first 1 m of the vadose zone was the dominant N loss pathway during flooding. Based on pore water sample and N2O emission data, denitrification played a lesser role in decreasing NO3- in the root zone but prolonged anoxic conditions resulted in a significant 29 % yield decrease in the 4-week flooded vineyard. The results from this research, combined with data from previous studies, are summarized in a new conceptual model for integrated water-N dynamics under Ag-MAR. The proposed model can be used to determine best Ag-MAR management practices