A comparison of forest and agricultural shallow groundwater chemical status a century after land use change

Prioritizing ecological restoration in hydrologically sensitive areas to improve groundwater quality

Greening is the optimal way to mitigate climate change and water quality degradation caused by agricultural expansion and rapid urbanization. However, the ideal sites to plant trees or grass to achieve a win-win solution between the environment and the economy remain unknown. Here, we performed a nationwide survey on groundwater nutrients (nitrate nitrogen, ammonia nitrogen, dissolved reactive phosphorus) and heavy metals (vanadium, chromium, manganese, iron, cobalt, nickel, copper, arsenic, strontium, molybdenum, cadmium, and lead) in China, and combined it with the global/national soil property database and machine learning (random forest) methods to explore the linkages between land use within hydrologically sensitive areas (HSAs) and groundwater quality from the perspective of hydrological connectivity. We found that HSAs occupy approximately 20 % of the total land area and are hotspots for transferring nutrients and heavy metals from the land surface to the saturated zone. In particular, the proportion of natural lands within HSAs significantly contributes 8.0 % of the variability in groundwater nutrients and heavy metals in China (p < 0.01), which is equivalent to their contribution (8.8 %) at the regional scale (radius = 4 km, area = 50 km2). Increasing the proportion of natural lands within HSAs improves groundwater quality, as indicated by the significant reduction in the concentrations of nitrate nitrogen, manganese, arsenic, strontium, and molybdenum (p < 0.05). These new findings suggest that prioritizing ecological restoration in HSAs is conducive to achieving the harmony between the environment (improving groundwater quality) and economy (reducing investment in area management).

Aquifer system and depth specific chemical patterns in fractured-rock groundwater from the Critical Zone revealed by untargeted LC-MS-based metabolomics

In the Earth's Critical Zone, water plays an essential role as a collector and transporter of metabolites and their transformation products. It is generally believed that the chemical profiles of groundwater are strongly impacted by land use. However, predictors for the effects of above-ground natural and anthropogenic activities on below-ground chemistry are rare. We reasoned that comparing groundwater metabolomes from different land-use sites and depths can give insight into this coupling of above and below-ground processes in the Critical Zone. This study used an LC-MS-based untargeted metabolomic approach to identify links between groundwater metabolomes from monitoring wells in fractured carbonate-/siliciclastic alternations along a hillslope of the Hainich Critical Zone Exploratory (CZE) in Thuringia, Germany. Our results identify the land-use type, aquifer system, and sampling depth as critical factors determining the differences among groundwater metabolomes. We established five groundwater metabolic clusters and correlated these to the aquifer systems, hydrogeochemistry, and microbial community composition. Our untargeted metabolomic approach reveals the limited connectivity of groundwater chemical profiles with above-ground activities and illustrates how deep the input signals can travel.

Nitrate dynamics in groundwater under sugarcane in a wet-tropics catchment

The transport of nitrogen (N) to groundwater and surface water in the form of nitrate (NO₃^-), as a by-product of the application of N-rich fertilisers, has been studied extensively. Yet, in the catchments adjacent to the Great Barrier Reef (GBR) in tropical north Queensland, Australia, NO₃^- transport in groundwater is not regularly monitored. An assessment of groundwater chemistry in the Liverpool Creek catchment of Queensland's wet-tropics region was conducted by regular sampling and analysis of groundwater over 12 months, through wet and dry seasons. A distinct spatial variability in groundwater chemistry was observed; groundwater aquifers with very low dissolved oxygen (DO) and NO₃^- consistently displayed relatively higher concentrations of sulphate (SO₄^2-), sulphur (S^2-) and ferrous iron (Fe²⁺) and low concentrations of dissolved organic carbon (DOC) (<2 ppm). Combined with averaged measured redox potential (Eh) of <250 mV, this indicates certain regions of the catchment have conditions favourable for removal of NO₃^- via autotrophic denitrification (DN), while other groundwater aquifers retained NO₃^– concentrations just above the acceptable trigger limits defined in regional water quality guidelines. Observations indicate that the naturally heterogeneous structure of the coastal alluvium contributes to the distinct variability in groundwater chemistry over small distances, with NO₃^- concentrations influenced by a combination of DN, lateral shallow drainage and potential adsorption to clay surfaces within the alluvial sediments.

Integrated Water Resources Research: Advancements in Understanding to Improve Future Sustainability

Anthropogenic and natural disturbances to freshwater quantity and quality is a greater issue for society than ever before. To successfully restore water resources in impaired watersheds requires understanding the interactions between hydrology, climate, land use, water quality, ecology, social and economic pressures. Current understanding of these interactions is limited primarily by a lack of innovation, investment, and interdisciplinary collaboration. This Special Issue of Water includes 18 articles broadly addressing investigative areas related to experimental study designs and modeling (n = 8), freshwater pollutants of concern (n = 7), and human dimensions of water use and management (n = 3). Results demonstrate the immense, globally transferable value of the experimental watershed approach, the relevance and critical importance of current integrated studies of pollutants of concern, and the imperative to include human sociological and economic processes in water resources investigations. Study results encourage cooperation, trust and innovation, between watershed stakeholders to reach common goals to improve and sustain the resource. The publications in this Special Issue are substantial; however, managers remain insufficiently informed to make best water resource decisions amidst combined influences of land use change, rapid ongoing human population growth, and changing environmental conditions. There is thus, a persistent need for further advancements in integrated and interdisciplinary research to improve scientific understanding, management and future sustainability of water resources.

Cadmium Background Levels in Groundwater in an Area Dominated by Agriculture

Cadmium is a highly toxic trace metal, which can be of geogenic or anthropogenic origin, for example, minerals, phosphate fertilizers, and combustion emissions. Due to its low sorption affinity compared to other heavy metals, Cd is easily mobilized, potentially resulting in elevated Cd concentrations in groundwater. This study assessed background levels of Cd in groundwater related to hydrogeology and hydrogeochemical processes through evaluation of a large hydrogeochemical data set composed of groundwater analyses from 6300 wells in Northwestern Germany. Calculated Cd background levels in groundwater were between 0.01 µg/L in hydrogeological units with mainly reducing conditions and 0.98 µg/L in less reducing groundwater recharge areas. The results showed that groundwater Cd concentrations above 0.5 µg/L (the German threshold value) are not necessarily elevated but could be the regional or ambient background level, depending on the hydrogeological unit. What would be considered as ambient background levels, however, indicated the influence by continuous intensive land use as well as the local geology, which is dominated by glacial deposits. Cadmium concentrations in groundwater were mainly controlled by hydrogeochemical and hydrogeological parameters and not by the amount of anthropogenic Cd input, in particular through the use of phosphate fertilizers. Instead, analyses of the solid phase revealed that Cd release from the aquifer matrix due to changes in hydrogeochemical parameters was more likely. Aquifer sediments in Northwestern Germany can be enriched in Cd originating from multiple sources, which in turn can cause elevated Cd concentrations in groundwater. Integr Environ Assess Manag 2019;00:1-11. © 2019 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals, Inc. on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Immobilisation of geogenic arsenic and vanadium in iron-rich sediments and iron stone deposits

Determination of how geogenic arsenic (As) and vanadium (V) is mobilised from naturally-enriched soils and iron (Fe) stones is integral for understanding the potential risk to the environment from changed land use conditions. Thus, the association of As, V and Fe in As-enriched sediments and Fe stones in Tertiary sediments of Melbourne, Australia, was assessed using chemical extraction methods, micro focused X-ray fluorescence and X-ray absorption spectroscopy. We show that the selective association of As with Fe during Fe stone formation has resulted in As enrichment of up to 60 times the concentration of surrounding soils, and 1000 times higher than mean As concentrations in world soils. In both soil and Fe stones, As was distributed with goethite as arsenate and relatively immobile under oxic conditions. The presence of V on the outer edge of the assessed Fe stone provided evidence of differences in historical As and V solubility; that is, As was immobilised by Fe during an earlier stage of Fe stone formation than V.

Characterization of sub-watershed-scale stream chemistry regimes in an Appalachian mixed-land-use watershed

An exploratory study was conducted in an urbanizing, mixed-land-use Appalachian watershed. Six study sites, characterized by contrasting land use/land cover, were instrumented to continuously monitor stream stage. Weekly grab samples were collected from each site and analyzed for elemental composition via spectrometric and spectrophotometric methods. Additional physico-chemical parameters were measured in situ. Data were analyzed using a suite of statistical methods, including hypothesis testing, correlation analysis, and principal components analysis (PCA). Significant differences (p < 0.05) between study sites were identified for every measured parameter except Co, Cu, Pb, and Ti concentrations. However, different parameters showed significant differences (p < 0.05) between site pairings. PCA results highlight consistent spatial differences between elemental composition and physico-chemical characteristics of streamwater samples. Results from correlation analyses indicated varying significant (p < 0.05) relationships between chemical parameters and hydroclimate metrics, with certain elements (e.g., Ca and Sr) and physico-chemical parameters (e.g., specific conductance) displaying greater sensitivity to hydroclimate at mixed-land-use sites, as compared to predominately urban, agricultural, or forest sites. Given the geological, topographical, and climatological similarities between the sites, and their close proximity, it was concluded that land use characteristics and associated hydrologic regime contrasts were the primary factors contributing to the observed results. Results comprise valuable information for land and water managers seeking to mitigate the impacts of land use practices on water resources and aquatic ecosystem health. The applied methodology can be used to more effectively target sub-watershed-scale remediation/restoration efforts within mixed-use watersheds, thereby improving the ultimate efficacy of management practices.

Improving understanding of mixed-land-use watershed suspended sediment regimes: Mechanistic progress through high-frequency sampling

Given the importance of suspended sediment to biogeochemical functioning of aquatic ecosystems, and the increasing concern of mixed-land-use effects on pollutant loading, there is an urgent need for research that quantitatively characterizes spatiotemporal variation of suspended sediment dynamics in contemporary watersheds. A study was conducted in a representative watershed of the central United States utilizing a nested-scale experimental watershed design, including five gauging sites (n=5) partitioning the catchment into five sub-watersheds. Hydroclimate stations at gauging sites were used to monitor air temperature, precipitation, and stream stage at 30-min intervals during the study (Oct. 2009-Feb. 2014). Streamwater grab samples were collected four times per week, at each site, for the duration of the study (Oct. 2009-Feb. 2014). Water samples were analyzed for suspended sediment using laser particle diffraction. Results showed significant differences (p<0.05) between monitoring sites for total suspended sediment concentration, mean particle size, and silt volume. Total concentration and silt volume showed a decreasing trend from the primarily agricultural upper watershed to the urban mid-watershed, and a subsequent increasing trend to the more suburban lower watershed. Conversely, mean particle size showed an opposite spatial trend. Results are explained by a combination of land use (e.g. urban stormwater dilution) and surficial geology (e.g. supply-controlled spatial variation of particle size). Correlation analyses indicated weak relationships with both hydroclimate and land use, indicating non-linear sediment dynamics. Suspended sediment parameters displayed consistent seasonality during the study, with total concentration decreasing through the growing season and mean particle size inversely tracking air temperature. Likely explanations include vegetation influences and climate-driven weathering cycles. Results reflect unique observations of spatiotemporal variation of suspended sediment particle size class. Such information is crucial for land and water resource managers working to mitigate aquatic ecosystem degradation and improve water resource sustainability in mixed-land-use watersheds globally.

Quantifying loading, toxic concentrations, and systemic persistence of chloride in a contemporary mixed-land-use watershed using an experimental watershed approach

A nested-scale experimental watershed study was implemented to quantify loading and persistence of chloride in an urbanizing, mixed-land-use watershed. A Midwest USA (Missouri) watershed was partitioned into five sub-basins with contrasting dominant land use. Streamwater was tested for chloride concentration four days per week from October 2009 through May 2014 at each site. Monitoring sites included co-located gauging and climate stations recording variables at 30-minute intervals. Results indicate significant (p<0.01) differences in chloride concentrations and loading between sites. Loading consistently increased from the forested headwaters (average=507kgday^-1) to primarily urban watershed terminus (average=7501kgday^-1). Chloride concentrations were highest (average=83.9mgL^-1) with the greatest frequency of acutely toxic conditions (i.e. 860mgL^-1) mid-watershed. This finding is in-part attributable to the ratio of chloride application to streamflow volume (i.e. increasing flow volume with stream distance resulted in chloride dilution, offsetting increased percent urban land use with stream distance). Results highlight the important, yet often confounding, interactions between pollutant loading and flow dynamics. Chloride peaks occurred during late winter/early spring melting periods, implicating road salt application as the primary contributor to the chloride regime. Floodplain groundwater analysis indicated seasonal sink/source relationships between the stream and floodplain, which could contribute to chronic toxicity and persistent low Cl^- concentrations in streamwater year-round. Results hold important implications for resource managers wishing to mitigate water quality and aquatic habitat degradation, and suggest important water quality limitations to stream restoration success in complex urban aquatic ecosystems.

Continuous and event-based time series analysis of observed floodplain groundwater flow under contrasting land-use types

There is an ongoing need to improve quantitative understanding of land-use impacts on floodplain groundwater flow regimes. A study was implemented in Hinkson Creek Watershed, Missouri, USA, including equidistant grids of nine piezometers, equipped with pressure transducers, which were installed at two floodplain study sites: a remnant bottomland hardwood forest (BHF) and a historical agricultural field (Ag). Data were logged at thirty minute intervals for the duration of the 2011, 2012, 2013, and 2014 water years (October 1, 2010-September 30, 2014). Results show significant (p<0.001) differences between Darcy-estimated groundwater flow at the two study sites. Although median flow values at the two sites were similar (0.009 and 0.010mday(-1) for the Ag and BHF sites, respectively), the BHF displayed a more dynamic flow regime compared to the Ag site. Maximum flow values were 0.020 and 0.049mday(-1) for the Ag and BHF sites, respectively. Minimum flow values were -0.018 and -0.029mday(-1) for the Ag and BHF sites, respectively. The BHF showed greater magnitude, longer duration, and more frequent negative flows, relative to the Ag site. Event-based analyses indicated a more seasonally responsive flow regime at the BHF, with higher flows than the Ag site during the wet season and lower flows than the Ag site during the dry season. Notably, the seasonal pattern of relative site flow differences was consistent across a wide range of precipitation event magnitudes (i.e. 8-45mm). Results are by majority attributable to greater rates of plant water use by woody vegetation and preferential subsurface flow at the BHF site. Collectively, results suggest greater flood attenuation capacity and streamwater buffering potential by the BHF floodplain, relative to the Ag, and highlight the value of floodplain forests as a land and water resource management tool.