Effects of subsurface barriers on seawater intrusion and nitrate accumulation in coastal aquifers

The subsurface barrier is one of various engineering measures used to prevent seawater intrusion in coastal regions which has been widely applied. However, its two common types, the cut-off wall and the subsurface dam, are both found to cause nitrate (NO3-) accumulation in landward aquifers. In this study, numerical simulations were conducted to investigate the mechanism of NO3- accumulation caused by the two types of subsurface barriers, as well as the influence of several key parameters, i.e. the infiltration NO3- concentration, the inflow DOC concentration, the barrier height and the barrier location on the performance of the subsurface barriers. The results showed that the cut-off wall generally requires a large height to ensure a satisfactory seawater prevention effectiveness, and it is more likely to cause NO3- accumulation compared to a subsurface dam. On the other hand, despite the subsurface dam may not result in the significant increase of NO3- concentration in groundwater upstream, it cannot be applied to the areas where SI has occurred due to the residual seawater problem. Moreover, the construction of a cut-off wall results in a stagnation zone appeared at the upper corner of the barrier, where accumulated NO3- significantly. With the increase of the barrier height, the stagnation zone expanded, leading to further increase of mean NO3- concentration in the landward aquifer. Since the construction of a subsurface dam will not generate such a zone, the subsurface dam generally has little impact on NO3- accumulation.

Avaliação do Potencial de Aproveitamento de Aluviões para a Construção de Barragens Subterrâneas no Semiárido Pernambucano

Diante da histórica escassez de recursos hídricos na região semiárida, é notória a necessidade de se encontrar novas fontes de obtenção e armazenamento de água, seja para o consumo humano e animal ou para produção agrícola. A implantação de barragens subterrâneas no semiárido brasileiro pode contribuir de forma positiva na melhoria do abastecimento humano em comunidades rurais, além do fornecimento de água para o consumo animal e para produção de alimentos. Como sua construção se dá sobre os solos aluviais, o presente estudo tem como objetivo avaliar o potencial de aproveitamento desses solos no estado de Pernambuco para a implantação de barragens subterrâneas e verificar as áreas passíveis de salinização para a escolha adequada desse tipo de intervenção. A área de estudo corresponde ao estado de Pernambuco, em que a presença de solos aluviais foi observada por bacia hidrográfica, utilizando os dados do Zoneamento Agroecológico do Estado de Pernambuco (ZAPE). Com a mesma base de dados foram selecionados os solos com características salinas e estimada, por aluvião, a capacidade de armazenamento de água. Foi possível verificar que o estado de Pernambuco apresenta consideráveis áreas de solos aluviais, com potencial para aproveitamento das águas subterrâneas e para construção de barragens subterrâneas, bem como identificar as regiões onde a implantação desse tipo de obra deve ser mais restritiva por conta do risco de salinização. Evaluation of the Alluvial Use Potential for a Construction of Underground Dams in the Semi-Arid Region of PernambucoA B S T R A C TGiven the historical scarcity of water resources in the semi-arid region, it is clear the need to find new sources for obtaining and storing water, either for human and animal consumption or for agricultural production. The establishment of underground dams in the Brazilian semi-arid region can contribute positively to the improvement of human supply in rural communities, as well as the supply of water for animal consumption and food production. As its construction takes place on alluvial soils, the present study aims to evaluate the potential use of alluvial soils in the state of Pernambuco for the implantation of underground dams and to verify the salinization areas for the appropriate choice of this type of intervention. The study area corresponds to the state of Pernambuco, where the presence of alluvial soils was observed by watershed, using data from Pernambuco State Agroecological Zoning (ZAPE). With the same database were selected soils with saline characteristics and estimated, in each alluvium, water storage capacity. It was possible to verify that the state of Pernambuco has areas considerable of alluvial soils, with potential for groundwater utilization and for the construction of underground dams, as well as to identify the regions where the implementation of this type of work should be more restrictive due to the risk of salinization.Keywords: groundwater, alluvium, subsurface dam, risk of salinization.

Impacts of Artificial Underground Reservoir on Groundwater Environment in the Reservoir and Downstream Area

Artificial underground reservoirs have changed the hydrological cycle from its natural condition. This modification may trigger a series of negative environmental effects both at the local and regional levels. This study investigated the impact of the Wanghe artificial underground reservoir on groundwater flow and quality in the reservoir and its downstream area. Wanghe is a typical artificial underground reservoir scheme in China, which assumes the dual function of fresh-water preservation and control of seawater intrusion. The groundwater flow pattern has changed after the reservoir construction, and the water level in the reservoir rose rapidly. Evaluation of long-term groundwater level fluctuation suggested that the reservoir deprived the downstream aquifer of the runoff, which it received under the natural flow regime. A preliminary isotopic evaluation using 3H was developed to understand the groundwater flow and renewal rates in the study area. The uniform distribution of tritium levels in the reservoir indicated that the stored water was well-mixed in both horizontal and vertical directions. The intervention on groundwater circulation also made differences in groundwater renewal rates between stored and downstream water. Field investigations on groundwater nitrogen pollution showed that the construction of the artificial underground reservoir resulted in nitrate accumulation in the stored water. Agriculturally derived nitrate was the largest contributor, and NO 3 − concentration varied considerably over time due to fertilization and irrigation activities, rainfall, and denitrification. NO 3 − -N distributed homogeneously in the reservoir, which was attributed to the construction of the subsurface dam, land use pattern and artificial groundwater flow.

Water resource management with a combination of underground dam/qanat and site selection of suitable sites using GIS

Subsurface dams and aqueducts have been used for many years as important sources of water in different parts of the world. Nowadays, little attention is paid to aqueducts. On the other hand, there is a great tendency for constructing and using subsurface dams in many arid and semiarid parts of the world such as Iran. A combination of these two structures was established in ancient Iran including Qanat Vazvan in Isfahan. Nowadays, new methods and techniques such as the Geographic Information System and Remote Sensing have been developed to process site selection of subsurface dams with high accuracy and in a short time. Study and research of the Qanat Vazvan structure emphasizes the idea that these two structures have been combined in a new way. Considering some main criteria and sub-criteria for subsurface dam site selection, with the existence of aqueduct, some suitable locations with a combination of aqueduct and subsurface dam in Pishkoh basin, Taft, Yazd province have been delineated. Some of the advantages of underground dams are their local use, being close to the demand sites and their capacity for gathering water in each season. These positive points when combined with reclamation of aqueducts (qanats) cause a renewable situation for qanats and their efficiency as a water resource management process. Underground dams can be built in a qanat's shaft by constructing some hatches inside the dam's wall. Hence, water can be stored during cold rainy seasons. The main and basic weaknesses of qanats can be covered through this combination. Hence, it causes a strong and optimum usage of qanat water.