GROUND WATER MONITORING

Ground water is a dynamic resource. For assessing its availability over time and space and preparation of management plans and ensuing sustainability of the development programmes, continuous or periodic monitoring of the behaviour of the system and quality regime are the pre-requisites. The principal variables likes water or head, chemical quality and temperature are required to be monitored .

Main objectives are

Detect early sign of over exploitation, water logging and/or other human activity on ground water levels.

Provide necessary information allowing for tailor made use and ground water resource management.

Provide information on ground water quality data.

Monitoring Network

Monitoring network is may be defined as a scientifically designed surveillance system of continuing measurements and observations, including evaluation procedures.
Following type of network can be distinguished for ground water quality and quantity.
Basic network (gives general information)
Benchmark station (Continuing series of consistent observations)
Specific network (known as impact stations, monitor influence of projects and water management system on ground water on more local scale)
Temporary network
Monitoring Ground Water Quality

Objectives of ground water monitoring are,

Basic data for general surveillance purpose.
Quality of abstracted ground water and impact of pollution.
To forecast the impact of possible pollution from known sources.
Water quality trends
Early warning system

Following types of network stations are used

Basic network
Specific network
Temporary network
Tools used in Water Level Measurements
Wetted tape
Mechanical sounder
Electrical water level indicator
Water level recorder
Electronic Recorders
Press log & Rain log
Pressure transducer
Dimensions of monitoring network
Dimensions has two broad aspects
Density of network stations
Sampling frequency.

Network Density

To begin with the main Hydrogeological units should be identified and limited number of stations should be established in each hydrological unit having homogenous water quality along the flow lines. For extending these additional stations are located along detailed cross sections of Hydrogeological systems.

Optimum number of observation wells (N) required estimating the average water level within their percentage error p is
N = (Cv)2/p
Where,
Cv - coefficient of variation
P - % of error
Additional number of wells required = N-n
N = (Cv/E)2
E - % error x range of variation of water level (R)/mean
water level
R - Maximum water level – Minimum water level

Frequency Of Monitoring System/Recording/Sampling

In general system having quickest response should be observed more frequently and frequency must be planned considering the timing of onset and cessation of monsoon, as the data of onset and cessation of monsoon are not constant throughout the country. A fair and reasonable approach would be to record the peak water level in the second half of September and last first half of November in area under influence of SW monsoon and in December and March in area influenced by NE monsoon.

Optimization Of Ground Water Observation Network

The economic and environmental benefits of appropriate and sustainable ground water quality/quantity management in regions with an excessive over exploitation of limited ground water resources may justify the casts of a dense network at stations, but the same number of stations in regions with abundant ground water resources and no quality problems may be considered as pure luxury.

For this optimization of ground water observation points is a must. The variance of krigging estimation error is a very powerful tool for optimization network.
s k2 = Var ( Zo* - Zo) =å l g (Xi , Xo) + m

The krigging variance depends only on semi variogram and the configuration of the observation point relation to the point estimated. It does not depend on observation value. Therefore of the semi variogram is already known the variance or standard deviation of estimation error for any particular water level observation point can be estimated. One can therefore conceptually add new fictitious measuring points, compute the map of the variance of estimation with this new point and compare with previous map. One can then locate the additional measurement points in the area where the variance of estimation error is high.

Conversely a network that is too costly to maintain can be reduced by maintaining only the observation points that gives the most acceptable map of estimation variance, in view of the objectives of observations i.e. general surveillance on local zone of interest.
Similarly the positions of the existing observation points can be re- adjusted to achieve optimum number and description of observation points in the network keeping in view the admissible krigging estimation error variance and thus the optimum observation network density for each unit can be decided.

Analysis Of Hydrographs

The ever increasing population and the demand for agricultural production created the necessity to exploit the locally available ground water resource. Which leads to over exploitation/increasing ground water development during the last couple of years. It has become urgent necessity to have careful monitoring of ground water regime. Keeping above in the view, a systematic monitoring of the ground water regime in time and space has become mandatory to manage the resource for its long term sustainability and planning for future generation.

Central ground water board has one of the objective is to monitor the ground water regime in a systematic manner, for creating an integrated database to analysis the data and formulate strategies to develop and manage the resource in sufficiently manner. This data facilitates to analysis of ground water regime and to estimate the ground water balance and water level fluctuation for time to time.

Ground Water System

In hydrological cycle the water is in a continuous motion, changes its form, place of occurrence with time and repeats the same activity in the cyclic manner. The details is deal in Chapter hydrological cycle and its relevance to ground water.

For Hydrogeologists it important to study the ground water system within zone of saturation, is a complex system of static and dynamic variables. The static variables are aquifer porosity, aquifers geometer and topography while the dynamic variables are rainfall, seepage from surface water bodies, evapotransipiration etc.

Controlling Factors

Ground water level is not static but it is dynamic due to under number of time dependent natural man made influences. This influence are involved by hydrological processes. The human activities like ground water withdrawal and artificial recharge processes negative (declining) and positive (rising) effects of ground water levels.

Recharge and Discharge Areas

In recharge areas is to note that water levels decreases with depth of piezometer setting in recharge areas. In the phreatic aquifer, stands at a higher level then the piezometric level in the piezometer. In the discharges areas, the water levels increase with the depth. The piezometeric surface stands above the phreatic surface directing the ground water to flow upwards from confined aquifer towards the phreatic aquifer.

Ground Water Hydrograph

Plotting of water levels, collected from observation wells and piezometers, against time of observation provide the information on this behaviour of water level with time and is termed as per ground hydrograph.
Fluctuation of Water Level with Time
The water level fluctuation are classified in to two category.
Long-term variation and
Short-term variation.
Long-term variation
Ground water level extending to several years analysed to study the long-term behaviour of water table.

Short Term Variation

Ground water level exhibit a short term effect of ground waster level by effecting change in atmosphere pressure, flooding streams, evapotranspiration, wind, oceans, tides, earth tides, earthquakes external loads etc.