Impact Assesment

Artificial recharge structures are constructed mostly with the objective of augmenting ground water resources and/or to improve its quality. Assessment of impacts of the artificial recharge schemes implemented is essential to assess the efficacy of structures constructed for artificial recharge and helps in identification of cost effective recharge mechanisms for optimal recharge into the ground water system. It also helps to make necessary modifications in site selection, design and construction of structures in future.

Impact assessment may require monitoring of the recharge structure, ground water regime, changes in pattern of water supply, cropping pattern, crop productivity and/or water quality. In recent years, tracers such as Tritium, Rhodomine B, fluorescent dye sand environmental isotopes are also being used for demarcating the area benefited by artificial recharge structures.

The methodology of impact assessment is highly site-specific and can vary considerably depending upon various factors such as hydrogeological set-up and ground water utilization pattern. General guidelines for impact assessment of artificial recharge structures are discussed briefly in the following sections.

1. Monitoring of Recharge Structures

Surface structures such as percolation ponds, check dams and cement plugs need to be monitored at regular intervals to assess the actual storage created in the structures, period of impounding, capacity utilization of the structure, rate of percolation and siltation problems if any. Quantification of storage in the structures may require setting up of monitoring devices within the structures. Devices such as gauges for area-capacity analysis are commonly used in surface recharge structures. Daily monitoring records are preferred for realistic assessment of storage created by multiple fillings of the structures. Evaporation and seepage losses from the structures are also to be accounted properly to evaluate the recharge efficiency of the structures.

In case of subsurface structures, the intake water supplied to the structures is measured by suitable measuring devices. Appropriate measuring devices such as flow meters and ‘V’ notches can be used for measurement. Daily records of such measurements help quantify the amount of water utilized for recharge purpose.

2. Water Level Monitoring

The objective of water level monitoring is to study the effect of artificial recharge on the natural ground water system. The monitoring system should be designed judiciously to monitor impact of individual structures which can further be extended to monitor the impact of groups of such structures in the area where artificial recharge is being done. Monitoring of water levels during the planning stage of artificial recharge projects helps in assessment of the ground water conditions of the area and helps in identification of the most suitable method for ground water augmentation. A properly designed observation well network is used for understanding the groundwater flow pattern and the spatial and temporal changes in water levels/potentiometric heads in the area.

During the planning and feasibility study stage, the observation well network is generally of low well density but spread over a large area with the primary aim of defining the boundaries of the aquifer to be recharged and to know the hydraulic characteristics of the natural ground water system. After identification of the feasible artificial recharge structures, the observation well network is redefined in a smaller area with greater well density.

For effective monitoring of the changes in the water levels due to artificial recharge, the network should have observation wells near the center of the recharge facility, at a sufficient distance from the recharge facility to observe composite effects and also near the limits of hydrological boundaries. If the aquifer being recharged is overlain by confining /semi-confining layers, piezometers should be installed to monitor the water levels of overlying and underlying aquifers separately to study the effects in both the aquifers. In cases where surface water bodies are hydraulically connected with the aquifers being recharged, it is advisable to monitor the water level profiles of both surface water and ground water.

Demarcation of the zone of influence of the artificial recharge structure is one of the main objectives monitoring in the context of artificial recharge projects. The following observations are generally associated with the area benefited by an artificial recharge structure:

1. Well hydrographs in the area benefited will have a flat apex during the period when there is water in the recharge structure (tank, pit etc.).
2. Wells located outside the zone of influence normally show an angular apex during the period of recharge, whereas those situated within the zone of influence have a flatter apex.
3. The recession limbs of well hydrographs close to a recharge structure normally have gentle gradients as compared to those located far off.
4. Crops in the zone of influence are normally healthier when compared to those outside the benefited area. Furthermore, crops with high water requirements are more likely to be grown in the zone of influence.
5. Well yields in the zone of influence will normally be higher when compared to those outside it. Wells in benefited zone may have more sustainability in lean period than those located outside.

The behaviour of water table / piezometric head profile prepared from the data collected from the observation well network over a period of time can clearly establish the efficacy of the artificial recharge scheme. Answers to questions related to the extent of the area benefited and the quantification of ground water augmentation could also be worked out from such data. The study of fluctuation overtime for both surface and ground water levels in the same area may also indicate whether the ground water augmentation is taking place as envisaged or not. In case any deviation is observed, the reasons for the same could be identified and necessary remedial measures taken up.

3. Water Quality Monitoring

A proper evaluation of potential water quality and aquifer quality problems associated with artificial recharge is a key component of a ground water recharge scheme. The development of reliable pre-, operational and post-operational monitoring programs is an integral part of the development of a successful ground water recharge scheme.

A reliable water quality monitoring system for an artificial recharge scheme will involve

• Evaluation of existing water quality data,
• Pre-Operational monitoring,
• Operational monitoring and
• Post-operational monitoring.

I. Evaluation of Existing Water Quality Data

The first step that should be followed in evaluating the potential water quality problems associated with a proposed ground water recharge project is to obtain detailed information on the chemical characteristics of the proposed recharge waters. A critical examination of the existing data on the waters that would be recharged to the aquifer should be made to first determine their reliability and representativeness. In case the available data is not considered to be reliable, collection and analysis of source water samples may be done afresh.

II. Pre-Operational Monitoring

The augmentation of recharge by surface waters and their associated contaminant scan greatly increase the potential for ground water quality problems due to the increased hydraulic and contaminant loading. The characterization of ground water quality is often not adequately done to properly evaluate potential ground water and aquifer quality problems associated with a ground water recharge project. it is important to properly assess how the variable parameters in sampling such as borehole volume purged and rate of purging before sampling influences the composition of the samples. Chemical parameters of particular importance in reliably assessing ground water quality samples are the redox conditions within the aquifer and the presence of suspended solids in the samples. Because of the chemistry of ferrous and ferric iron, small changes in the redox (oxidation reduction) characteristics of the sample as a result of the introduction of oxygen into the sample during sampling can drastically change the chemical characteristics of the samples. Hence, it is important to maintain the oxygen concentrations in a sample collected from an aquifer the same as that of the aquifer. Failure to do so could readily change the distribution between dissolved and particulate forms of many trace contaminants of water quality concern.

The presence of suspended solids in a water sample from an aquifer is a clear indication that the sampling well has been improperly constructed and developed and/or the sampling procedure used, especially the purging, has been improperly done. Aquifers typically do not contain large amounts of suspended material. Aquifer samples that contain suspended material are unreliable to properly characterize chemical characteristics of the ground waters within the aquifer at the point and time of sampling.

It is also important that the sampling program for the ground water is properly developed to reflect the site specific hydrogeology of various principal components of the aquifer. Failure to do so could readily lead to erroneous conclusions concerning the chemical characteristics of the aquifer waters and the chemical reactions that can take place within the aquifer upon introduction of recharge waters to it. Depending on the situation, at least one year and often several years of data may be needed to reliably characterize the aquatic system of interest. The best way to determine the length of time necessary in pre-operational monitoring as well as the frequency of monitoring a particular system is to examine the ability to predict the chemical characteristics of the system prior to collecting the next set of samples. Once it becomes clear that the characteristics of a particular recharge water source and aquifer are predictable with a high degree of certainty based on past monitoring results, it should then be possible to reduce the frequency and duration of pre-operational monitoring. If, however, it is not possible to make these predictions reliably because of the high variability in the systems, proceeding with the operation of the proposed recharge project could be met with significant problems in detecting incipient water quality problems before they adversely impact large parts of the aquifer.

III. Operational Monitoring

With the initiation of the recharge activities, a significant increase in the frequency of sampling, especially near the point of recharge, should occur. Actually the operational sampling program should be initiated several months before actual recharge starts in order to evaluate the ability to conduct the monitoring program with the facilities and personnel available. If the pre-operational monitoring program has been passive, the nit should, at the time of initiation of recharge, become an active program, where the data is examined in detail as soon as it is available for the purpose of determining its reliability and any potential problems that are developing with the recharge project. In addition to chemical and microbiological measurements in the recharge waters as well as within the aquifer, detailed monitoring of the hydraulic characteristics of the injection / infiltration system should be conducted to determine the changes in the hydraulic characteristics of the recharge system and the aquifer in the vicinity of the recharge. In addition to monitoring the chemical contaminants in the recharge waters as well as aquifer, consideration should be given to the contaminant transformation products that might be formed in the recharge water. An area of particular concern in the recharge waters is whether there is sufficient BOD in these waters to exhaust the dissolved oxygen in the aquifer waters for those aquifer systems that are oxic prior to initiation of recharge. Bore hole dissolved oxygen measurements should be made at frequent intervals at various distances from the point of recharge in order to detect incipient dissolved oxygen depletion that could lead to its exhaustion from the recharge waters. Since, in general, except for nitrate-related issues, anoxic conditions in aquifers tend to lead to poor water quality, care should be taken to prevent the recharge waters from becoming anoxic within the aquifer. Failure to do so could readily result in iron, manganese and hydrogen sulphide problems. If problems of this type start to develop, it may be necessary to add dissolved oxygen either directly or through the introduction of hydrogen peroxide, in the recharge waters in order to prevent problems of this type from occurring.

Once the operational monitoring program data have become stabilized, i.e. are predictable based on past monitoring results, then the frequency of operational and post-operational monitoring can be decreased. This will likely take several years of operation, however, for fairly constant composition recharge waters and fairly homogeneous aquifer system with respect to its hydrogeologic and chemical characteristics.

Impact Assessment

Manual on Artificial Recharge of Ground Water 138 The type of water quality monitoring programme depends on the specific problem being studied, such as changes in ground water quality, effect of soil salination, prevention of any contamination etc. The samples to be collected will also depend on the purpose and are generally categorized into a) Indicative, b) Basic and c) Comprehensive. Indicative samples are collected at 1 to 4 months intervals and are used to ascertain the presence of recharged water in the aquifer. Basic samples are taken at monthly intervals for wells already influenced by recharge to determine the effect of recharge on ground water quality and the purification provided by flow through the soil and aquifer system. Comprehensive samples are taken at intervals of 6 months to 1 year for observation wells and production wells to determine water quality with respect to specific standards for intended water use.

IV. Post-Operational Monitoring

When groundwater recharge is terminated, it is important that the monitoring of the aquifer be continued until the waters in the aquifer stabilize in composition. This will normally take several years of monthly monitoring. This monitoring should continue for quarterly intervals for several years.