Geophysical Survey : An essential tool for Rain Water Harvesting

The main reason for a rapid decline in the water table can be attributed to our ever-increasing exploitation of ground water resources for meeting the growing water demands of agriculture, domestic and industrial purposes. Increasing urbanization and industrialization have also resulted in ground water pollution causing adverse effects on the health, environment and imbalance in the ecosystem.

The basic purpose of artificial recharge of groundwater is to replenish water into aquifers that have been depleted due to excessive ground water extraction.

Artificial groundwater replenishment systems involve techniques that modify the natural movement of surface water and utilize suitable civil construction techniques in order to address issues such as:

  • Enhancement of the sustainable yield in areas where over-development has depleted the underground aquifers. Storage and conservation of excess surface water for evolving future requirements
  • Improvement in the quality of existing ground water through dilution.
  • Avoiding flooding of roads during storm showers by capturing the rainfall run-off which would otherwise overwhelm sewer or storm drains.
  • Help reduce soil erosion and flood hazard.
  • Provide an eco-friendly method of water resource conservation.

In this post we discuss the importance of performing a Geophysical Survey of an area targeted for ground water recharge or extraction. We have presented sample data from our records showing variations in soil formation with changes in location and how a Geophysical Survey can help in planning & designing structures for groundwater extraction and artificial ground water recharge.

GEOPHYSICAL SURVEY 

In order to know the ground water condition , thickness of alluvium, depth to rock, the extent and depth of aquifer and quality of ground water, Geophysical Vertical Electrical Soundings are conducted in the specific areas. The results of these investigations and their interpretation forms the basis of identification of sites for construction of rain water harvesting structures.

Objectives of a Geophysical Survey:

  • Determine the depth and thickness of saturated aquifer zones.
  • Quantify the expected alluvium thickness and depth to rock at the sites.
  • Determine the variation of quality of water with depth.
A Geophysical Survey In Progress
A Geophysical Survey In Progress

Investigation Methodology

The Geoelectrical resistivity technique uses an artificial current source whereby a low frequency (4HZ) current is used. Controlled amount of current is introduced into the ground through current electrodes and measurements are carried out with the help of potential electrodes.

The current and potential electrodes are placed in various configurations, but the most extensively used electrode configuration for subsurface investigation is the Schlumberger Configuration. In this configuration the four electrodes are placed symmetrically along a straight line, the current electrodes are on the outside and the potential electrodes on the inside along the array.

With this configuration, to change the depth range of the measurements, the current electrodes are displaced outward. When the ratio of the distance between the current electrodes to that between the potential electrodes becomes too large i.e. more than 5 times, the potential electrodes must also be displaced outward, otherwise the potential difference becomes too small to be measured with sufficient accuracy.

In the Schlumberger Configuration, the apparent resistivity (Pa) is calculated by the formula.

Where, Pa is apparent resistivity.
L is half of the distance between current electrodes.
l is the half of the distance between potential electrodes.
π is constant,
∆v is potential difference and
I is amount of current.
K is a constant known as Geo-electric factor and based on the type of electrodes configuration.

Geophysical measurements are based on the assumption that the subsurface consist of a sequence of distinct layers of finite thickness, each of these layers is assumed to be electrically homogeneous and isotropic and the boundary planes between subsequent layers are assumed to be horizontal.

The resistivity data are interpreted using the Schlumberger Sounding Data Processing and Interpretation Programme.

For ascertaining ground water levels, the resistivity response depends primarily on the amount of impregnating water, the conductivity and quality of water and manner in which water is distributed. The first two factors have a nearly linear relation with the resistivity while the influence of the third factor is more complicated and depends on the nature of aquifer material.

In summary, it can be stated that a dry soil formations, whether porous or non porous are practically poor conductors and hence the resistivity will vary with amount of pores and quality of water. The chemical quality of ground water corresponds with aquifer resistivity.

 LIMITATIONS:

There are certain inherent limitations in estimating water withdrawal levels and lithology, which are being mentioned as under:-

1.  Scientific and human error possibilities tothe extent of 15% cannot be ruled out both in quantitative and qualitativeassessment of ground water.

 2.Increased ground water development activity in and around the area investigatedmay also effect rate of subsurface flow of water in the area in the years tocome.

Thus, any recommendations for construction of structures (Tube- well/ Dug cum bore well) for ground water development are based on indirect science of Geophysical investigations.

Presented below are data from 3 different sites to show different soil formations calling for appropriate modifications in rain water harvesting designs.

Artificial Ground Water Recharge: A Challenging Endeavor

Science is the pursuit of understanding the principles of Nature through a systematic study and observation of the processes, reactions and systems that constitute the physical and natural world.

Our enhanced understanding of natural principles and phenomenon has been instrumental in helping scientists and innovators bring about rapid scientific advancements and technological breakthroughs across many fields.

While our understanding of the geophysical and natural processes happening below the Earth’s surface has been steadily increasing in the past few decades, some crucial technological breakthroughs in Rain Water Harvesting have still not been achieved.

The process of Rain Water Harvesting involves critical interplay of numerous natural factors like the multiple unique properties of water, force of gravity, atmospheric pressure etc. along with various soluble and insoluble elements and compounds naturally present in the soil of the target location.

Due to the limited understanding of the natural processes taking place beneath the surface along with inadequate investigative tools developing artificial ground water recharge techniques still remains a challenging and complex exercise.

We have been emphasizing the fact that rain water harvesting is certainly not just about digging a hole in the ground and releasing water into it, as some people think it to be. The key to success of any rain water harvesting structure is the understanding and experience of the harvester. There is no margin for miscalculation that may disturb the critical natural balance and lead to stagnation of water on the surface or cause risk of damage to buildings and structures nearby.

We often see construction of impractical designs that are not in line with the principles of artificial ground water recharge. It should come as no surprise that the number of dead & defunct rain water harvesting structures far exceeds the number of the alive & operational systems, not just in India but the world over.

We are battling an almost constant water scarcity prevailing in most parts of India and people start coming under water stress within a couple of months after the monsoon. The time has come to address our water crisis at top priority.

We at SILVERON keep motivating people to value the rain and harvest the water before it runs off. Very often the rain water is even diverted into external storm water drains and sewerage lines without remorse. It is not in our hands to control rainfall but it is surely in our hand to prepare to receive, manage and harvest the rain water whenever it falls- wherever it falls – in whatever quantity it falls.

Drying lake bed. Native American Proverb.
We do not inherit the Earth from our ancestors, we borrow it from our children

While we had an excellent monsoon with above average rainfall in 2019, lot of urban areas lost the surplus water through runoff or floods. When it comes to the environment, we cannot gamble with the future of our next generation. Planning and implementation of rain water harvesting and water conservation requires a sustained effort which we must act upon on a priority basis.

At SILVERON, with decades of field experience in experimenting, designing and installing thousands of effective Rain Water Harvesting structures across India, we also recognize the multitude of factors that make artificial ground water recharge challenging.

Our attention to detail, understanding site-specific conditions and vast experience developing solutions across various topographies are just some of the reasons that make SILVERON the premier rain water harvesting solution provider in India and reflects in the flawless performance of SILVERON recharge structures.

Why do some Rain Water Harvesting systems fail?

When water is not absorbed by a Rain Water Harvesting (RWH) system and it stagnates around the structures or if the structure itself collapses, we consider it as a failure of the RWH system.

There are innumerable designs and ideas floated on the internet for Rain Water Harvesting. We evaluated several designs and have found that many of these have inherent shortcomings which can cause RWH systems to fail. We observed that these designs have not been properly tested in real-world scenarios over a period of several monsoons or their designers simply do not report failures.

Through our decades of experience designing & developing Rain Water Harvesting solutions, we recognize the multitude of factors which can lead to failures of Rain Water Harvesting structures.

The idea of this post is not to comment on the design of any individual person or entity but to select a few basic reasons which can defeat the entire RWH effort.

To illustrate these scenarios, we have randomly picked up two designs for Rain Water Harvesting structures commonly circulated on the internet, which in our experience are not successful in practical applications.

a. RWH systems using hollow cement rings with slots

This design proposes using hollow cement rings with holes to construct a Rain Water Harvesting bore up to a depth of 10 feet to 30 feet, with a diameter of about 3 feet approximately. The hollow cavity of the bore stacks cement rings of diameter 2.5 to 3 feet over each other from the base till the top. The top is covered with a slab as you see in the pictures below

Hollow cement rings with slots
Hollow cement rings with slots

Expectation:  The designers of such a system imagine that the rain water will fill the hollow space in the pit and the water will pass through the slots in the cement rings and when it comes in contact with the soil surrounding the cement rings, the water will be absorbed by the soil.

Reality:  In practice, while the rain water fills the empty space in the pitit thereafter moves out of the holes in the cement rings and forms a watercolumn outside the rings. Now as the soil which has come in contact with thewater dissolves in this water column, this mixture of mud and clay re-entersthe hollow space and tries to fill the hollow space with soil.

This processcontinues as and when the remaining empty space is filled with water. Thisprocess stops once the empty space in the rings is completely filled andcompacted by the adjoining soil.

Collapsed Hollow Cement Ring Based RWH Structure
Collapsed Hollow Cement Ring Based RWH Structure

At this stage,any chance of ground water recharge stops due to compaction of soil. The soilwhich has moved in through the holes in cement rings leaves a hollow space adjoiningthe structure causing it to eventually collapse around the structure as isvisible in the photo above.

This fundamentally flawed design increasesthe risk of soil shifting and structural damage to nearby constructions.

b. RWH Systems using percolation Bore-Pits

This design proposes creation of percolation bore-pits for of rain water absorption through creation of 15 to 30 feet deep bores where the top end is enclosed in a 2 x 3 feet deep bore-pit covered with a perforated RCC slab as shown in the image below.

Percolation Bore Pit Design
Standard Percolation Bore-Pit Design

Expectation: The designers here imagine that the flowingrain water over paved surfaces (such as streets or parking lots) will effortlesslyfall into the chamber through the perforated RCC cover slab and thereaftersettle around the bore-pit to eventually be absorbed by the bore.

Reality: Having observed a practical implementation of this design on one of our sites, we recognized that when water flows over a perforated slab it forms a film of water and thereafter most of the water flows and passes over. Some rain water laden with silt and clay enters the perforation and deposits in the pores in the shape of a cone further reducing the opening size of the hole on the inside of the slab and finally the hole gets completely blocked.

Clogged Percolation Covers With Silt
Perforated cover slab choked by silt

Further, below the cover in the bore-pit in the silt-removal chamber:

Expectation: It is imaginedthat the flowing rain water that reaches the silt removing chamber will gentlyfall on the coarse sand and all the silt and clay suspended in the rain waterwill be restricted by the coarse sand from moving further down. After this onlyclean water will pass through the pebbles and enter the recharge pit pushingthe air out through the air vent and letting the soil absorb the water.

Reality: During rainfallthere is massive turbulence in water leading to the water appearing muddy andalso there is no time for decantation – silt mixed with clay and small pebbles flowswith rain water and fill the chamber to the brim as intake speed of anyrecharge structure is relatively slow.

The turbulence in the chamber also disturbs and suspend the coarse sand inthe water. This water with suspended silt, clay, coarse sand and small pebbles formsa paste which tries to find a passage into the air vent chokes the air ventpipeshown below the perforated cover within the chamber.

Once the air-vent pipe gets choked, the recharge process completely stopssince choking of the air vent pipe is like somebody closing a pipette with athumb to stop the water column from falling down.

Other factors like the shallow depth of pit, chances of choking of recharge bore pit by infiltrating coarse sand mixed with silt and clay entering with rain water into the bore hole as well through the pebbles, the unmentioned size of pebbles and unclear relevance of the diameter of the pit are likely to jeopardize the performance of this structure design.

The success of SILVERON rain water harvesting designs is a result of decades of untiring experimentation and experience in field work across diverse topographies.

At SILVERON, Learning & Innovation are the heart of what we do.

Rain Water Harvesting: Where does the buck stop?

Those working in bureaucratic work environments such as large government departments or corporate offices may have often observed a unique phenomenon of passing the buck around and seeing those who do so get rewarded along the way.

This cycle of offloading critical decision making by delegating it to committees of subordinates working on stringent deadlines with vague objectives is ultimately only a way of shedding responsibility. The objective of passing the buck around is to avoid taking risks and as a consequence never being wrong.

This short-sighted system empowers those who do not take real risks but are experts in asking others. In this system, if a task succeeds & the risk pays off – those who perform it get much less credit while those who allotted the task claim all the credit.

Coming to our subject WATER – we observe a similar pattern.

At a time when almost the entire country and specially major metros like Delhi, Hyderabad, Bangalore  & Chennai are reeling under severe water stress and scarcity, our civic bodies & water resource management agencies seem to forget that they owe us an explanation as to why such a situation crops up year after year and what measured have they taken in the previous years to prevent such a dire situation?

Newspaper Info-graphic on India's impending Water Crisis
Newspaper Info-graphic on India’s impending Water Crisis

Nature has been kind to us with the stronger than expected monsoon season in 2019. The plentiful rainfall in this season has at least given us some respite from the most severe crisis experienced this year. However, the water crisis & dire warnings around depleting ground water levels are a wake-up call to preserve our water resources.

We need to develop systems to manage, harvest and conserve rainwater which could be used later in times of scarcity.

Interestingly, when it comes to conserving and harvesting rainwater, our civic bodies and municipal authorities are passing the buck around on to ordinary citizens to make immediate arrangements to recharge rainwater or else in some cases even risk fines.

We must realize that the number of defunct rain water harvesting systems all over the country far outnumber the perfectly working systems simply because Rainwater Harvesting is not as simple as it may look. Rainwater harvesting and ground water recharge require development of site-specific designs based on the survey of local conditions, soil analysis and study of nearby structures and topography.

Ironically, when pressure is exerted by the civic bodies on ordinary citizens to construct and show their rain water harvesting systems, the first thing that happens is that inexperienced contractors come up with impractical or incomplete designs that are not in line with the principles of artificial ground water recharge process. At times people themselves try to simply make cheap structure as an eye wash.

We have observed that even some of the rain water harvesting structure designs provided by the civic authorities are only theoretical and have apparent flaws as these designs have not been time-tested over many monsoons.

All this chaos leads to absolute failure of the purpose of the regulation and wastage of resources. The civic bodies & water-supply departments that extract ground water and supply water to industries, homes and institutions have not even setup recharge infrastructure to the replenish the quantity of water for which it has billed the public.

Public accountability should require that our municipal agencies declare the total quantity of water pumped out along with the recharge potential created by them.  

When there are piezometers in every area our electricity and water bill should also mention the ground water level in the billing area so that people know their water table and the gravity of the situation.

The government agencies at Center, State and local levels should engage with experts and develop designs for large-scale public infrastructure for rain water harvesting using scientifically designed and tested methods.

The success of SILVERON rain water harvesting designs is a result of decades of untiring experimentation along with experience and learning using scientific methods.

Using roads as catchment for ground water recharge

Every monsoon we see familiar scenes of flooding in our towns and cities overwhelming our urban infrastructure, inundating our roads and highways with huge quantities of water for days. On the other hand, for remaining months of the year our cities have to fight a never-ending battle against water scarcity and depleting water tables.

This imbalance is a man-made crisis caused by concretization of surfaces, filling up of ponds and lakes for urban land-use, increasing density of building infrastructure and reduction of green spaces around our cities.

How can we address this imbalance?

A good starting point is to re-phrase the problem of flooding in our cities. Rather than consider it only as a storm-water drainage issue we should consider using our roads and highways as water catchment areas for ground water recharge.

The massiveness of the roads & highway infrastructure as rain water catchment area would be astonishing once we take into account the length and breadth of this catchment. While water scarcity is looming large in most parts of India we have comfortably preferred to ignore the hundreds of thousands of square kilometers of ready in-hand catchment.

Using our urban infrastructure to also help recharge our ground water reserves is a sustainable approach with many benefits however, it requires careful study to be designed and implemented correctly.

Some people are of the opinion that rain water falling on the roads should not be recharged since there are contaminants like rubber remnants from friction of tires and oil spillage on roads due to vehicular traffic.

Surface contaminants from vehicular traffic
Surface contaminants from vehicular traffic

Some engineers avoid rain water harvesting beside the road due to fear of road collapse due to shifting of soil.

A road cave-in during the monsoons
A road cave-in during the monsoons

Some people wonder if it is even possible to hold back, guide and recharge the rain water falling on roads and highways.

Typical water-logging during monsoon rains

To begin with, we at SILVERON firmly believe that every drop of rain water must be prevented from running off long distances on the road, must be prevented from evaporation and must be recharged into the ground close to where it falls.

There are surely some contaminants on the roads but most of them are not water soluble and also during the rains the dilution levels are extremely high hence we should not lose out on this opportunity.

Recharge of rain water along these highways also support the idea of recharging rain water where ever it falls thus benefiting the entire area at large.

It must be underscored that recharging rain water close to a road is a highly specialized work since there is a risk of shifting of soil from under the road into the rain water harvesting structure leading to development of hollow space below the road which may not be visible at the first instance but may cause caving over time creating risk for commuters.

We at SILVERON have years of experience in building rain water harvesting and ground water recharge structures that are designed to perform alongside roads. Through are experience, we have following suggestions to offer:

  • Highways should have a proper slope on both sides from the center for water to immediately flow towards the edge of the road. This will not only prevent the road from damage but will put the water into shallow storm water drain running along road’s edges.
  • Storm water drain should have baffle walls a regular intervals. and these drains may not be covered and instead filled with 40 mm gravel to prevent any paper trash, poly bag, cloth etc. from chocking the drain while allowing the water to easily enter it.
  • A SILVERON design recharge shaft should be constructed on the outside of the drain preferably between two baffle walls and connected to the drain .
  • The recharge shaft design has to be modified such that the water from the drain is released into the recharge shaft bore sufficiently below the ground level so that it can percolate deeper into the ground. This will not disturb the compaction of the road.
  • Restaurants, shops or petrol pumps abutting the highways should ensure that they put slabs to protect these drains from getting clogged with sand or trash.

SILVERON designed catchment systems collect rain water run off from the road into the storm water drains. The gravel in these drains filters the water and prevents trash like polythene bags, paper etc from chocking the drain. Rain water percolates into the drain and moves through the connecting pipes to be recharged by the recharge shaft.

When conserving rain water, we just need to have the will that creates the way.