Seasonal variation in groundwater levels and quality under intensively drained and grazed pastures in the Montagu catchment, NW Tasmania

Water quality is a significant environmental issue in the Montagu River and its estuary in north-west Tasmania. Groundwater is the major contributor to baseflow for about half of the year. ‘Hump and hollow’ surface drainage is increasingly being used to reduce the effects of seasonal waterlogging on pasture production. However, little is known about the effects of ‘hump and hollow’ structures on watertable levels or intensive grazing on groundwater quality in the catchment. The objectives of this study were to evaluate the impacts of ‘hump and hollow’ drainage by comparing watertable levels in drained and undrained paddocks and to quantify the effects of intensive grazing on groundwater quality underlying pastures.In December 2004, 10 wells and 2 piezometers were installed at depths of 2-6 m at seven sites along two transects across the dairying area of Togari. Water levels were monitored and water samples collected every 2 months were analysed for pH, electrical conductivity, total dissolved solids, ammonium, nitrate, nitrite, total nitrogen, dissolved reactive phosphorous, Ca, Mg, K and Na. Thermotolerant coliforms and Enterococcus were measured when watertable levels were low and high.Watertable levels were within 0.5 m of ground level for over 3 months on undrained sites. ‘Hump and hollow’ surface drainage increased the depth of the unsaturated zone under the ‘humps’ but did not lower the watertable. Watertable levels on the crests of the ‘hump and hollow’ structures rose and fell daily in response to periods of rainfall and drought. Gradients of the groundwater surface, albeit very low, indicated the potential for groundwater flow from the base of the hills to the Montagu River in the centre of the valley.The median nitrate concentration of all samples was 0.018 mg NO₃-N L⁻¹ but one site had nitrate concentrations in excess of that recommended for potable water for a period of 1-2 months. Nitrate concentrations varied seasonally by 20-1000 times with an early winter pulse of nitrate evident both in the groundwater and in the Montagu River. In contrast, the median ammonium concentration in the groundwater was 0.274 mg NH₄-N L⁻¹ which was well above the trigger value for lowland streams. The median concentration of dissolved reactive phosphorus was 0.008 mg P L⁻¹ which was slightly higher than the trigger value. There was some evidence of low levels of faecal bacterial contamination of the shallow aquifers.Transects across the dairying area did not clearly demonstrate increasing concentrations of analytes due to intensive grazing though lower levels of nutrients were generally found at sites adjacent to undisturbed native forest. Variation in water quality parameters along the transects suggested water quality at a site was mostly related to local conditions and hazards.     

Identification of nitrate leaching hot spots in a large area with contrasting soil texture and management

Identification of nitrate (NO₃) leaching hot spots is important in mitigating environmental effect of NO₃. Once identified, the hot spots can be further analyzed in detail for evaluating appropriate alternative management techniques to reduce impact of nitrate on groundwater. This study was conducted to identify NO₃ leaching hot spots in an approximately 36,000 ha area in Serik plain, which is used intensively for agriculture in the Antalya region of Southern Turkey. Geo-referenced water samples were taken from 161 wells and from the representative soils around the wells during the period from late May to early June of 2009. The data were analyzed by classical statistics and geostatistics. Both soil and groundwater NO₃-N concentrations demonstrated a considerably high variation, with a mean of 10.2 mg kg⁻¹ and 2.1 mg L⁻¹ NO₃-N for soil and groundwater, respectively. The NO₃-N concentrations ranged from 0.01 to 102.5 mg L⁻¹ in well waters and from 1.89 to 106.4 mg kg⁻¹ in soils. Nitrate leaching was spatially dependent in the study area. Six hot spots were identified in the plain, and in general, the hot spots coincided with high water table, high sand content, and irrigated wheat and cotton. The adverse effects of NO₃ can be mitigated by switching the surface and furrow irrigation methods to sprinkler irrigation, which results in a more efficient N and water use. Computer models such as NLEAP can be used to analyze alternative management practices together with soil, aquifer, and climate characteristics to determine a set of management alternatives to mitigate NO₃ effect in these hot spot areas.     

Nitrate leaching assessment in a long-term experiment under supplementary irrigation in humid Argentina

Applying high rates of nitrogen (N) fertilizer to crops has two major disadvantages: (1) the low N fertilizer use efficiency and (2) the loss of N by leaching, which may cause groundwater nitrate (NO₃⁻) pollution, especially in humid areas.The objectives of this study were to adjust and validate the LEACH-W model simulations with data observed in the field; to quantify nitrate concentrations in the soil solution; to estimate N loss by leaching; and to determine the moments during the year when greatest nitrate transport events occur beyond the rooting profile.A randomized complete block design with four replications was established on a typic Argiudoll. Crop fertilization treatments consisted of three N rates (0, 100, and 200 kg N ha⁻¹) using urea and ammonium nitrate solution (UAN) as the N source. Corn (Zea mays L.) was planted and ceramic soil-water suction samplers were installed to depths of 1, 1.5 and 2 m. Drainage was estimated by the LEACH-W model, which adjusted very well the actual volume of water in the soil profile. Nitrogen losses were statistically analyzed as repeated measure data, using the PROC MIXED procedure.Losses of nitrate-nitrogen (NO₃⁻-N) during the study increased as the rate of N applied increased. At all depths studied, statistically significant higher values were found for 200 N compared to 100 N and 0 N, and for 100 N compared to 0 N (p < 0.001).The greatest NO₃⁻-N losses through leaching occurred during crop growth. Significant differences (p < 0.05) were found between cropping and fallow in the three treatments and depths studied for seasons 4 and 5; these two seasons produced the highest drainage volumes at all depths.     

Multi-method assessment of nitrate and pesticide contamination in shallow alluvial groundwater as a function of hydrogeological setting and land use

In this study deterministic, multivariate and stochastic methods are applied to a combined temporal and spatial monitoring data set, in order to assess nitrate and pesticide levels and contamination risk in shallow groundwater. The case study involves an area in the Mondego River alluvial body in central Portugal, where agriculture is the main land use, with predominantly maize, rice and some vegetable crops supported by river water irrigation. Factorial correspondence analysis (FCA), reducing the original data matrix to a small number of independent orthogonal factors, is applied to detect associations between nitrate levels, land use (crop type), lithology and groundwater depth. Indicator-geostatistical techniques are used to create maps indicating the probability of nitrate concentrations in groundwater exceeding predetermined threshold values, including the drinking water standard (98/83/EC) and vulnerable zone designation criterion (91/676/EEC) of 50 mg/l NO₃⁻. For pesticides the leaching potential is determined by calculating the Groundwater Ubiquity Score (GUS), based on the sorption coefficient and soil half-life for each pesticide compound. Results for nitrate show an overall very low risk of exceeding 50 or 25 mg/l, whereas the risk of exceeding 9.5 mg/l (third data quartile) is particularly high in areas where FCA shows correlation of nitrate contamination with vegetable and maize crops, aerobic conditions, lower groundwater levels and to some extent, coarser grained sediments. On the contrary, nitrate levels under rice are lowest and correlated to a reduced environment, finer-grained sediments and a higher water table. Denitrification is found to be an important attenuation process, as well as dilution by surface water irrigation and precipitation. Crop type and irrigation source are seen to have a large influence on the nitrate contamination potential of groundwater. Total concentrations of the analysed pesticide compounds above the regulatory limit of 0.5 μg/l are observed in 32% of the analysed water samples, with a maximum value of 16.09 μg/l. The probability maps provide a particularly interesting example of how multiple-well monitoring results over a certain period can be condensed into single maps and used by water engineers, managers and policy-makers.     

Hydrochemical and stable isotopic assessment of nitrate contamination in an alluvial aquifer underneath a riverside agricultural field

Major ions and stable isotopic (δD_water, δ¹⁸O_water, δ¹⁵N_nitrate, δ¹⁸O_nitrate) measurements in concert with hydrochemical modeling were used in order to elucidate the sources and geochemical processes controlling nitrate contamination of shallow alluvial groundwater underneath a riverside agricultural field in the Buyeo area, Korea. Beneath vegetable fields in the sandy soil, the mean nitrate concentration of groundwater was 148.6 mg/L, which is significantly higher than in groundwater (mean 28.8 mg/L) beneath silty soils underneath rice paddy fields. Nitrogen isotope data indicate that synthetic fertilizers are the predominant source of groundwater nitrate in the study area. Denitrification during recharge through rice paddy soils appears to be responsible for the lower nitrate concentrations in groundwater beneath the silty soil zone. The relationship between nitrogen and oxygen isotope data of nitrate also suggests mixing of two different groundwater bodies with nitrates from the silt zone and the sand zone. Geochemical mass balance modeling on hydrochemical data indicates that various agricultural chemicals such as urea, lime, magnesium sulfate and potassium chloride dissolve in vegetable fields of the sandy zone, resulting in significant enrichment of various solutes such as K⁺, Ca²⁺, Mg²⁺, NO₃⁻, SO₄²⁻ and Cl⁻. As a consequence of over-utilization of synthetic nitrogen fertilizers, the sand zone is characterized by very high nitrate concentrations in the groundwater. This study suggests that a reduction of over-fertilization especially on vegetable fields in the riverside sand zone is required to minimize the nitrate contamination of groundwater. This study also shows that combination of geochemical and isotopic techniques with simple mass balance modeling provides information about the causes and processes of nitrate contamination of groundwater underneath a riverside agricultural field. The study also provides sustainable measures to optimize fertilization rate as an important basis of eco-friendly agriculture.     

Nitrate contamination of a rural aquifer and accumulation in the unsaturated zone

Groundwater contamination was studied in a rural setting of the Upper Pantanoso Stream Basin (UPSB) in the southeast of Buenos Aires Province, Argentina, where potential contaminant sources include inorganic fertilizer. Nitrate–N concentrations, greater than accepted level for safe drinking-water of 10 mg l⁻¹ were present in 36% of sampled wells and 67% of samples had nitrate concentrations exceeding the background level of 5 mg l⁻¹. Temporal fluctuation of nitrate concentrations in the groundwater was attributed to seasonal fluctuations in recharge and plant growth. Nitrate concentration was measured in deep soil profiles to determine the extent of leaching. Nitrate accumulation in the unsaturated zone of a soil cropped with potatoes was three times higher than the baseline N concentration found in the pasture. The greatest nitrate concentration in the soil profile occurred under irrigated corn where excessive nitrogen was applied. These results show that high fertilization rates and irrigation lead to increased hazards of groundwater pollution.     

Carbon and nitrogen cycling in a tropical Brazilian soil cropped with sugarcane and irrigated with wastewater

Carbon (C) and nitrogen (N) dynamics in agro-systems can be altered as a consequence of treated sewage effluent (TSE) irrigation. The present study evaluated the effects of TSE irrigation over 16 months on N concentrations in sugarcane (leaves, stalks and juice), total soil carbon (TC), total soil nitrogen (TN), NO₃⁻-N in soil and nitrate (NO₃⁻) and dissolved organic carbon (DOC) in soil solution. The soil was classified as an Oxisol and samplings were carried out during the first productive crop cycle, from February 2005 (before planting) to September 2006 (after sugarcane harvest and 16 months of TSE irrigation). The experiment was arranged in a complete block design with five treatments and four replicates. Irrigated plots received 50% of the recommended mineral N fertilization and 100% (T100), 125% (T125), 150% (T150) and 200% (T200) of crop water demand. No mineral N and irrigation were applied to the control plots. TSE irrigation enhanced sugarcane yield but resulted in total-N inputs (804-1622 kg N ha⁻¹) greater than exported N (463-597 kg N ha⁻¹). Hence, throughout the irrigation period, high NO₃⁻ concentrations (up to 388 mg L⁻¹ at T200) and DOC (up to 142 mg L⁻¹ at T100) were measured in soil solution below the root zone, indicating the potential of groundwater contamination. TSE irrigation did not change soil TC and TN.     

Assessment of groundwater quality and its variations in the capture zone of a pumping well in an agricultural area

Agricultural activities are frequently associated with water contamination. The spreading and storage of fertilizers, for instance, may result in groundwater contamination due to pollutants leaching into an aquifer. Nitrates and fecal bacteria are two important contaminants associated with agriculture. Thus, the development of efficient strategies for groundwater protection in agricultural areas requires an assessment of these two contaminants. Given this perspective, groundwater quality monitoring was carried out over the whole capture zone of a municipal well located in an agricultural area in the St.-Lawrence Lowlands in Québec. Thirty-eight piezometers were installed within the roughly 2 km² capture area of the well to measure physico-chemical parameters such as major ions, field measured parameters (pH, electrical conductivity, dissolved oxygen, water level, temperature), and isotopic ratios, bacteriological parameters (Heterotrophic Plate Count—HPC, enterococci, total coliforms, Escherichia coli) and their variations in space and time. Groundwater was sampled from the pumping well and the piezometers during 25 field campaigns in 2005, 2006 and 2007. The results demonstrate the impact of agricultural activities on nitrate contamination. They indicate high spatial and temporal variations in nitrate concentrations, from 6 to 125 mgNO₃⁻/L within the capture area, with 40% of the samples exceeding the Québec drinking water limit of 45 mgNO₃⁻/L. Nitrate pollution in the municipal well exceeded 45 mgNO₃⁻/L during 2005, but no bacteriological contamination was observed. The results also show a high variability of nitrate concentration with depth within the capture zone. Electrical conductivity appears as a good indicator of the presence of nitrate and calcium ions in this capture zone. Correlations between nitrate, calcium and chloride suggest that these ions come from the same source of fertilizer. Nitrate isotopic composition suggests that nitrate in groundwater originates from both chemical and organic fertilizers. The bacteriological results show that the extracted volume of water during sampling of a piezometer has a significant impact on the bacteria count. The variability of bacteriological pollution is important in space and time, showing a higher contamination during summer. Only 2% of the raw water samples exhibit contamination exceeding the drinking water standard for treated water. Total coliforms seem to be a good precursor of E. coli or enterococci contamination. Globally, the physico-chemical and bacteriological groundwater quality within the studied capture area and the pumping well shows contamination by nitrates, but low contamination levels by fecal bacteria.     

Optimized denitrification bioreactor treatment through simulated drainage containment

In the design of wood-based, enhanced-denitrification bioreactors to treat nitrate in agricultural drainage, the consideration of the highly variable flow rates and nitrate concentrations inherent to many drainage systems is important. For optimized mitigation of these nitrate loads, it may be best to contain drainage water prior to treatment in order to facilitate longer, more constant retention times rather than to allow cycles of flushing and dry periods in the denitrification bioreactor. Simulated containment prior to bioreactor treatment compared to passing drainage directly through a bioreactor was investigated with the use of six pilot-scale denitrification bioreactors constructed with plywood and filled with Pinus radiata woodchips at Massey University No. 4 Dairy Farm (Palmerston North, New Zealand). Initial bromide tracer tests were followed with a series of five simulated drainage events each at successively declining inflow nitrate concentrations. During each drainage event, three pilot bioreactors received a simulated hydrograph lasting 1.5 days (Non-Containment treatment) and three pilot bioreactors received the same total drainage volume treated over 4 days at a constant flow rate (i.e. constant retention time; Containment treatment). Results showed significantly different total mass removal efficiencies of 14.0% vs. 36.9% and significantly different removal rates of 2.1 g N m⁻³ day⁻¹ vs. 6.7 g N m⁻³ day⁻¹ for the Non-Containment and Containment treatments, respectively, which indicated that treating drainage at constant retention times provided more optimized nitrate removal. While this work was done to evaluate treatment under New Zealand drainage conditions, it also provides valuable information for optimizing agricultural drainage denitrification bioreactor performance in general.     

Policy incentives for reducing nitrate leaching from intensive agriculture in desert oases of Alxa, Inner Mongolia, China

Excessive irrigation and nitrogen applications result in substantial nitrate leaching into groundwater in intensively cropped oases in desert areas of Alxa, Inner Mongolia. An integrated modelling approach was developed and applied to compare policy incentives to reduce nitrate leaching. The integrated model consists of a process-based biophysical model, a meta-model, a farm economic model and an assessment of policy incentives. The modelling results show that there are “win-win” opportunities for improving farm profitability and reducing nitrate leaching. We found that 4471 Yuan ha⁻¹ of farm gross margin could be obtained with a reduction in nitrate leaching of 373 kg ha⁻¹. Farmers’ lack of knowledge about water and nitrogen in soil, and on crop requirements for water and nitrogen could explain the differences, so that agricultural extension is an appropriate policy incentive for this area. When the economic optimum is obtained reductions in nitrate leaching are not achievable without profit penalties and there is a “trade-off” relationship between farm profitability and groundwater quality protection. The combination of low elasticity of nitrate leaching and large elasticity of farm gross margin against water price increases results in very high costs for reducing nitrate leaching (105.6 Yuan kg⁻¹). It is suggested that if the water price increases were coupled with subsidies for adopting nitrate leaching mitigation practices, environmental gains could come at a lower cost.     

Modelling seawater intrusion in the Burdekin Delta Irrigation Area,North Queensland,Australia

The Burdekin Delta is a major irrigation area situated in the dry tropics of North Queensland. It is unique in that (i) it overlies shallow groundwater systems that serve as a major water supply for the irrigation of sugarcane, and (ii) it is adjacent to the world heritage listed Great Barrier Reef. Water management practices include large recharge pits and surface spreading of water to assist with replenishment of the groundwater. This has been useful in maintaining groundwater levels to help control seawater intrusion. This technique, however, can be costly and ineffective in unconfined aquifer systems, which are subjected to large amounts of groundwater pumping for irrigation. There are more than 1800 production bores currently used for irrigation in the Burdekin Delta and the large volumes of water extracted have at times lowered the regional water tables and made it difficult to control seawater intrusion.     

Sensitivity analysis and field testing of the RISK-N model in the Central Valley of Chile

We present the results from a sensitivity analysis and a preliminary short-term, site-scale performance assessment of the analytical soil and groundwater nitrate transport RISK-N. The study was carried out in the Central Valley of Chile, on a 2.6 ha corn (Zea mays L.) field underlain by a shallow unconfined aquifer during the cropping season 2000–2001. Nitrogen levels in soils as well as NO₃⁻–N irrigation water and groundwater concentrations were monitored through the crop-growing period, the latter by a network of 16 monitoring wells. A sensitivity analysis shows that both the nitrate flux from the vadose zone and NO₃⁻–N groundwater concentration are mainly influenced by the initial soil nitrogen levels, water input, and soil porosity. Also, simulated groundwater NO₃⁻–N levels are sensitive to changes on the saturated zone denitrification constant. An additional analysis further reveals the significance of the latter parameter, in conjunction with the amount of applied nitrogen fertilizer. We obtained a good agreement between observed average and simulated values. While the model performs well when spatially averaged values are used (root mean square error, RMSE = 1.4 mg l⁻¹ of NO₃⁻–N), the prediction error increases (RMSE = 1.9 mg l⁻¹ of NO₃⁻–N) when the concentration in each well is considered. This fact could be explained by the time and space scale of the experiment and the characteristics of the RISK-N model. The model is easy to use and seems appropriate for mid- and long-term studies of nitrogen contamination in groundwater for agricultural conditions in the Central Valley of Chile and under limited field data availability conditions. However, it needs to be tested for longer periods and under different climatic conditions, soil types, and aquifer characteristics, before its range of applicability can be fully established and recognized.     

Assessing ground water quality in the irrigated plain of Triffa (north-east Morocco)

Irrigated agriculture in (semi-)arid regions may exert serious pressures on groundwater resources and jeopardise further agricultural and socio-economical developments. For assessing these pressures, we present in this paper results from a groundwater quality survey performed in 2005 within the irrigated agricultural Triffa plain in north-east Morocco. The study focuses on the physico-chemical and bacteriological quality of the groundwater body within the plain and exploits the correlation and spatial dependency of the quality parameters. It is demonstrated that the water quality in this region is critical. Nitrate levels are situated between 2 and 153 mg/l, with 73% of the observations exceeding the critical level of 50 mg/l. Nitrite, ammonia, orthophosphate and dissolved organic matter content do not exceed existing norms. Bacteriological residues (faecal, total coliforms, faecal streptococcus and clostridium sulfido-reductants), however, are retrieved in nearly all water samples. Bacteriological contamination is merely correlated with nitrite and ammonia content rather than with nitrate content, indicating a possible contribution of local pollution sources to groundwater deterioration. The variability of the nitrate and bacteriological pollution is important and spatially correlated. The spatially dependency is modelled using spherical and Gaussian semi-variograms and is used to map the nitrate and bacteriological contamination using ordinary kriging techniques. The results shown are significantly different as compared to earlier studies on groundwater quality for the studied aquifer. The differences may be explained partially by modified but inappropriate fertilizer management practices in combination with intensive irrigation. Given the agricultural developments in this area, further deterioration of the groundwater quality is expected if no mitigation strategies are developed.     

Observations on soil and groundwater contamination with nitrate: A case study from Lebanon-East Mediterranean

The impact of agricultural practices on soil-groundwater quality in the sub-humid Bekaa plain of Lebanon-East Mediterranean was monitored in four fields (F) between July 2007 and July 2009. These were occupied by continuous mint (F1), summer potato/wheat/potato (F2), lettuce/lettuce/potato/wheat/summer potato (F3) and table grapes (F4). N input calculated on a two-year basis, was in the following ascending order F4, F2, F3 and F1. Soil samples, analyzed down to 200 cm depth, showed high nitrate and chloride concentrations at the end of the 2007 and 2008 seasons. Soil chloride and nitrate peaks recorded in October 2007 and 2008 disappeared below 200 cm overwinter. The calculated N biannual discharge ranged from 130 (F4), to 516 (F2), to 778 (F1), to 879 kg ha⁻¹ (F3). Groundwater quality was studied in 21 wells distributed along a sequence stretching from the Litani River to the eastern water dividing line. Based on the nitrate concentrations, the well located at the top of the water dividing line was the only one suitable for drinking purposes. Eight wells were mildly contaminated, therefore suitable for irrigation purposes except for sensitive crops. Twelve wells, positioned in the plain, showed a nitrate level exceeding 200 mg L⁻¹. Protecting the soil and groundwater quality is a top priority to maintain the ecological and agricultural functions of water.     

Simulation of nitrate leaching under potato crops in a Mediterranean area. Influence of frost prevention irrigation on nitrogen transport

The Sa Pobla area (Majorca Island, Spain) heavily depends on the use of groundwater resources for irrigation and urban water supply and is characterised by the presence of intensive potato farming activities. The Plioquaternary aquifer is unconfined and contains high levels of nitrate concentrations. To analyse the risk of contamination to the aquifer arising from agricultural practices, the amount of water and nitrogen leached below the root zone was simulated by the GLEAMS code. Data for model calibration and validation were obtained from field experiments on six potato crops for the years 2004-2007.When air temperatures drop below 1 °C irrigation water is applied to prevent crops from frost damage. During times of anomalously low air temperatures, the risk of nitrate leaching is increased by as much as 318% from frost prevention irrigation under normal local conditions.The GLEAMS simulation model was successfully calibrated for Sa Pobla conditions under potato cropping as shown by RMSE values for the water transport module of 0.19, 0.14 and 0.13 for the calibration period and 0.20, 0.25 and 0.15 for the validation period at depths of 0.3, 0.6, and 0.9 m respectively; and for the chemical transport module the R² value was 0.82 for the calibration period and 0.60 for the validation period. Consequently, for Sa Pobla conditions, GLEAMS can be used to assess the effectiveness of different agricultural management practices to reduce nitrate leaching. It was concluded that additional irrigation water applied for frost prevention plays a very important role in nitrate leaching below the root zone, which enhances the nitrogen loading to the aquifer.     

Simulation of nitrate leaching under irrigated maize on sandy soil in desert oasis in Inner Mongolia, China

Water scarcity and nitrate contamination in groundwater are serious problems in desert oases in Northwest China. Field and ¹⁵N microplot experiments with traditional and improved water and nitrogen management were conducted in a desert oasis in Inner Mongolia Autonomous Region. Water movement, nitrogen transport and crop growth were simulated by the soil-plant system with water and solute transport model (SPWS). The model simulation results, including the water content and nitrate concentration in the soil profile, leaf area index, dry matter weight, crop N uptake and grain yield, were all in good agreement with the field measurements. The water and nitrogen use efficiency of the improved treatment were better than those of the traditional treatment. The water and nitrogen use efficiency under the traditional treatment were 2.0 kg m⁻³ and 21 kg kg⁻¹, respectively, while under the improved treatment, they were 2.2 kg m⁻³ and 26 kg kg⁻¹, respectively. Water drainage accounted for 24-35% of total water input (rainfall and irrigation) for the two treatments. Nitrogen loss by ammonia volatilization and denitrification was less than 5% of the total N input (including the N comes from irrigation). However, 32-61% of total nitrogen input was lost through nitrate leaching, which agreed with the ¹⁵N isotopic result. It is impetrative to improve the water and nitrogen management in the desert oasis.     

Contamination of shallow groundwater system and soil-plant transfer of trace metals under amended irrigated fields

This study focuses on experimental pilot assessment of contamination of shallow groundwater systems and soil-plant transfer of trace metals under amended irrigated fields. The study approach involved a pilot experimental (greenhouse) set-up of organo-mineral amended test plots/troughs (40 cm × 47 cm × 46 cm) planted with two common vegetable crops (Amaranthus hybridus and Abelmoschus esculentus) and irrigated with wastewater. In addition to the geochemical analyses of the primary un-amended and amended soils before planting as well as residual soils after harvesting, measurements of the physico-chemical parameters and chemical analyses of trace metals concentrations in irrigation leachates and harvested vegetable tissues were also undertaken following appropriate standard sample preparation and analytical methods.The results of the geochemical analyses carried out on irrigation leachate samples collected during the sprouting stage revealed that most of the analyzed trace metals in the collected leachates exhibited 2-10 folds depletion (except for Cu and Co with enrichment of about 1.5-3 folds) compared to the initial wastewater used for irrigation. A situation attributed to uptake/bioaccumulation of these metals and selective enrichment in the residual soils as well as to leaching by infiltrating irrigation water. Nonetheless, the observed higher trace elements concentrations in the second sets of leachates collected during harvesting stage compared to the first sets of leachates collected during the sprouting/vegetative stage is an indication of higher plant uptake during sprouting/vegetative stage or initial sorption/complexation of biosolids amendment before later vertical re-mobilization by infiltrating irrigation water.Although, virtually all of the analyzed metals exhibited elevated concentrations (2-173 ppm) in both A. hybridus and A. esculentus, a closer evaluation revealed 1.2-8.2 folds enrichment of Cr, Co, Ni, Cd, Cu, and Pb in A. esculentus compared to that of A. hybridus, an indication of the fact that phyto-accumulation of trace metal is plant-specific and dependent on physiological set-up. The overall evaluation had clearly demonstrated the potential danger of bioaccumulation of toxic trace metals under biosolid amended soils as well as impacts of irrigation-induced leaching on the shallow groundwater quality, while the need to evolve a sustainable agricultural practices is also highlighted.

Capsule

Organo-mineral amendment can lead to trace metal bioaccumulation (in plants) and irrigation-induced leaching to shallow groundwater system.     

Nitrate fate in a Mexican Andosol: Is it affected by preferential flow?

Andosols are the dominant soils in the Valle de Bravo basin, the origin of a significant amount of Mexico City's drinking water. The main land use is agriculture and most of the existing surface water bodies are eutrophic. Nitrogen fertilizer is used extensively. There have been very few studies on nitrate (NO₃⁻) fate in this type of soil and region. Comprehensive laboratory studies were conducted to determine the fate of NO₃⁻ in an Andosol profile from Valle de Bravo, in order to assess the risk of water resources contamination. Nitrate retention was analysed statically (using batch experiments) and dynamically (using intact and packed soil columns) at different soil depths and its competition with Cl⁻ was evaluated. Complementary laboratory experiments were conducted to study water transport through the columns and nitrogen transformations in the soil. In batch and columns, NO₃⁻ adsorption was linear in the range of concentrations studied and higher in the deepest soil layer. Preferential flow pathways were found in the unaltered deeper soil layers, while tillage activity in the top layer destroyed the pore continuity. In spite of the deeper soil layer's greater capacity for NO₃⁻ retention, the presence of preferential flow pathways coupled with high rainfall intensities, makes the NO₃⁻ mobile below the root zone at 1 m depth and increases the risk of groundwater contamination. The results illustrate the complexity of nitrate fate in Andosols and can be used to improve agricultural practices in the central Mexico region.     

Runoff, erosion, and water quality of agricultural watersheds in central Navarre (Spain)

Two experimental watersheds, La Tejería (1.69 km²) and Latxaga (2.07 km²), appointed by the Government of Navarre (Spain) for assessing the effect of agricultural activities on the environment, were monitored during 10 years (1996-2005). Both watersheds are roughly similar with regard to soils, climate (humid sub Mediterranean) and land use (almost completely cultivated with winter grain crops). The first results for both sites on runoff, exported sediment, nitrate and phosphate are presented.Most runoff, sediment, nitrate and phosphate yields were generated during winter, when variability was also the highest of the whole year.La Tejería had much higher sediment concentrations and sediment yield than Latxaga. Nitrate concentrations were also significantly higher at La Tejería, with values constantly over the critical threshold (>50 mg NO₃ l⁻¹). However, phosphate concentrations were similar in both watersheds and corresponded to water with a significant risk of eutrophication. Differences in watershed behaviour could be mainly due to differences in morphology, topography, and amount of stream channel vegetation between both sites.This is an unprecedented research for the region and the generated dataset is of paramount importance for research issues such as hydrology, erosion and water quality. The results highlight the complexity of Mediterranean agricultural landscapes and the need for further analyses to better ascertain the processes behind them.     

Effects of drip irrigation systems on the recovery of dissolved oxygen from hypoxic water

The dissolved oxygen concentration (DOC) is an important irrigation water quality parameter that can become a limiting factor in some intensive agriculture systems. A low DOC in the irrigation water may have critical consequences because it causes root oxygen deficiency, which in turn can result in agronomic problems. The aim of this study was to improve the understanding of the dynamics of the DOC in hypoxic irrigation water when passing throughout a surface drip irrigation system (DIS) and seeping into the soil. To this end, an experimental DIS consisting of three types of commercial emitters and a venturi air injector, installed in-line, was set up for evaluation. Furthermore, subsurface water samplers were buried to catch the water in the soil. The trials were conducted with water from two different sources. The control treatment was performed with fresh channel water, which had a high DOC (7.54 mg L⁻¹; 92.2% saturation), and the low DOC treatments were supplied from a covered agricultural reservoir and had DOC values less than 1.08 mg L⁻¹ (10.8% saturation). After the low DOC treatments, the final DOC in the soil 24 h after irrigation ranged from 3.77 mg L⁻¹ to 5.31 mg L⁻¹ (47.2% to 65.2% saturation). There was an increase in the DOC in all stages of the experimental DIS, which was more important in the water passing through the emitters. The main factor determining the final DOC was the type of emitter, where DOC differences were correlated to their flow performance. The control treatment reached a similar DOC in the soil 24 h after irrigation, indicating that using hypoxic water under DIS does not affect the final soil DOC. Finally, the application of a venturi air injector increased the DOC in the low DOC source up to values typically found in open channels and reservoirs.