Terrain Influence on Groundwater

Physiography, Geological setting and subsurface features such as aquifer properties control the occurrence and movement  of groundwater in a region.

Physiographically, Kerala divides into Lowland, Midland and Highland regions .  The lowland falls in the altitude range of 0–7.5 m and occupies an area of 3979 km2.  Characterized by coastal plains, lagoons and comprises beaches, dunes, flood plains, river terraces, and marshes. The midland region falls in the altitude range of 7.5–75 m and occupies an area of 16231 km2. It consists of dissected peneplains with numerous flood plains, terraces, valley fills and colluviums.  The highland above an altitude of 75 m occupies an area of 18654 km2.

Texturally, in the lowlands, sandy soil dominates in the beaches and dunes and clayey soils in the runnels. Estuaries and backwaters have loamy and clayey soils with high salt concentrations. Soils of the midland, lateritic in nature, predominantly clayey or gravelly clayey. The soil environment of the state favors moderate to high infiltration of water.

Geologically, Kerala preserves the major units of the Archaean continental crust, such as granulites, granites, gneisses and greenstones . The southern part of the state exposes stigmatized meta-sedimentary and meta-igneous rocks. The rocks towards the central part  include charnockites and other gneissic rocks. The northern part contains migmatitic gneisses and occasional patches of granulites, schists and granites and Crystalline limestone bands . . Both marine and non-marine rocks of the Neogene period fringe the coastal tract of Kerala in two major basins of deposition between Trivandrum and Ponnani and Cannanore and Kasaragod. 

 Physiography, the state  divides into six groundwater provinces namely, the coastal sandy belt, coastal alluvial tract, laterite formation, inter-mountain valley-fill area, sedimentary formations, and crystalline terrain.

Occurrence of Groundwater 

The most widely spread aquifers of the state are the Crystalline Rock Aquifers (CRA). The shallow aquifers of crystalline rocks , made up of highly  or partly decomposed weathered and fractured rocks. Thick weathered zones seen along the midland area either beneath the laterites or exposed. In the hill ranges, a thin weathered zone seen along  the topographic lows with less area elevation and gentle slopes.

The depth to water level in this aquifer varies from 2 to 16 m below ground level (bgl), and  yield of the well ranges from 2 to 10 m3 per day. The potential fractures in the crystalline rocks  at depths ranging between 60 to 175 mbgl with yield varying from less than 1 to as much as 35 liters per second (lps) . Another important groundwater zone – Tertiary Rock Aquifers (TRA) where water occurs under phreatic conditions in the shallow zone and under semi-confined  conditions in the deeper zones. 

Monitoring 

Monitoring of groundwater regime over time is necessary to gather information  and  for quality through  managing the groundwater resource. Done by establishing groundwater monitoring network stations to record the response of  groundwater regime to  natural and anthropogenic stresses of recharge and discharge parameters with reference. The natural conditions affecting the regime involve climatic parameters like rainfall, evapotranspiration, etc., whereas anthropogenic influences include, pumpage from the aquifer, recharge due to irrigation systems, and other practices. In Kerala, groundwater monitoring is carried out by the Central Ground Water Board (CGWB) and the State Ground Water Department (SGWD).

Groundwater resource potential

The occurrence and availability of groundwater vary considerably from place to place within the state depending on the prevailing climate, geomorphological and hydrological conditions.

Groundwater Level during Different Seasons

The groundwater level and its fluctuation mostly depend on the hydro-geological conditions of the area as well as the topography, rainfall patterns, withdrawal, etc. 

The pre-monsoon water level in Kerala ranges from minimum of 0.02 to maximum of 55.05 mbgl. 88.41% of monitoring wells exhibit water level varying from 0.10-10.0 mbgl. The coastal tracts and also the eastern parts of the high ranges in Idukki district exhibit shallow water levels. The areas falling in the midland region generally show water level in the range of 2-10 mbgl. In Kasaragod, Kollam and Thiruvananthapuram districts, shows deep water level in the range of 20–55.05 mbgl  and  thick lateritic overburden .

During the month of August, the depth of water level varies from minimum of 0.14 mbgl to  maximum of 25.56 mbgl, but 92.4% monitoring wells exhibit a water level variation between 0.10 and 10.0 mbgl. The well water level in the coastal tracts does not indicate any major change from  April, but the depth of water level in areas with thick lateritic overburden comes down to 20–26 mbgl. 

During the post-monsoon period , the depth to water level mostly falls within the range of 0–10 mbgl . Along the coastal tracts , midland areas of Palakkad districts as well as the northeastern parts of Idukki district, the water level is shallow, less than 2 mbgl . The midland areas generally show water levels in the range of 2–10 mbgl, but in the central part of Kasaragod district, the northern parts of Kannur district, and in certain areas of Wayanad district, the water levels in the range of 10–20 mgbl. The areas with thick lateritic overburden exhibit deep water level in the range of 20–53.6 mbgl.

During January, the depth of water level varies from near ground level to 24.70 mbgl, but 90% of monitoring wells exhibit the range of 0–10 mbgl. Shallow water level of less than 2 mbgl , seen along the coastal tracts , midland areas of Palakkad district and northeastern parts of the high ranges . The midland areas show water level in the range of 2–10 mgbl. The central parts of Kasaragod district, northern parts of Kannur district, and certain areas of Wayanad district, reveal water levels  in the range of 10–20 mbgl. 

Level Fluctuation and Trend

Comparison of the water levels in November with that of April – indicates  rise in the range of 0.01–9.97 m. Major part of the state recorded a  rise in the water level  as revealed by 80% of observation wells. Long term  fluctuations in groundwater can be studied by comparing pre-and post-monsoon water levels. 

Estimation Procedure

 It’s done by modified GEC-97 norms by the state level committee constituted by the Government of Kerala (GoK). Two approaches used for the estimation procedure : Water Level Fluctuation (WLF) method and  Rainfall Infiltration Factor (RIF) method. In the WLF method  the concept of storage changes between various input and output components. Input refers to the recharge from rainfall , other sources and  subsurface flow into the unit of assessment. Output refers to groundwater draft, evapotranspiration, base flow to streams and subsurface outflow from the unit. The RIF method essentially uses three parameters, namely the Rainfall Infiltration Factor, Specific Yield and the Unit Groundwater Draft. 

Groundwater development 

 Process in which the available resource extracts for different uses.The available resources – static reserve and dynamic reserve. In static reserve groundwater contained  within the permanently saturated zone of groundwater reservoir.  The dynamic reserve represents the long-term average annual recharge over the static reserve under conditions of maximum groundwater use. The static reserve forms an assured source over which the dynamic reserve replenishes annually. Groundwater development  extract only the dynamic reserve, for resource sustainability. It’s done by appropriate extraction structures depending on the quantum of water required, geologic and hydrogeologic conditions and economic considerations. The conventional structures used for groundwater development are dug wells, boreholes, tubewells, filter-point wells, step wells, tunnel wells and spring capping. 

 Groundwater Abstraction Structures

Dug wells

Traditional and most common method of abstracting groundwater in areas where the water table is shallow. The depth of dug wells varies generally from 2-10 m but in coastal and river beds it shows less than 1m and in thick laterite overburden areas it could be above 20 m.  With 1.5 m diameter, it will provide adequate working space for construction.  As the storage space increases, the diameter of the well also increases.  Aquifer types determine the performance of dug wells in terms of quantity .

Wells with  large diameter and depth expose  greater area for infiltration , fast recharge and provide larger storage spaces. Generally protected with a well head to prevent the direct flow of water from the ground surface. The well shaft between the ground surface and water table  seales with brick  for preventing collapse of side walls. If the well shaft portion is a hard laterite or crystalline rock formation it indicates  low chances of collapse .  Easy to construct using local skill , materials and  generally used for small water demands. 

Borewells

Narrow wells of 15 cm diameter drilled in crystalline rocks to tap water from weathered zones having considerable thickness and/or fractured zones with high density of fissures. Generally drilled using down the hole hammer (DTH) or by auger boring.6 .  These wells collect water from deeper aquifers often up to several hundred meters. The top portion is protected with a case of steel or PVC pipe to prevent the collapse of loose soil or rock into the well and also to avoid seepage of water saturated in the surface or near-surface zones. Water tapped using submersible pumps and the yield depends on the hydraulic properties of the aquifer. Therefore  after construction, a pump test is done for determining the safe yield

Tubewells 

Narrow wells of diameter 15–30 cm drilled in sedimentary rock formations to tap water from medium to deep aquifers. Drilled generally by rotary drilling machines often up to several hundred meters. Auger drilling machines also use, if  aquifer depth not deeper than around 25 m. Rotary drilling machines use a segmented steel drilling string, typically made up of 6 m sections of galvanized steel tubing threaded with a bit or other drilling device at the bottom end. The rotary drilling machines help to install  steel casing (well assembly) into the well in conjunction with  drilling of the actual bore hole. Air and/or water inside acts as a circulation fluid to displace cuttings and cool bits during the drilling.

Another form of rotary drilling, termed mud rotary, makes use of a specially made mud (Bentonite clay)to create enough hydraulic pressure to hold the side walls of the tubewell. The well assembly is a casing pipe for tubewell which seals the layers and facilitates seepage of water .And  designed in such a way according to the depth and thickness of various aquifer zones  by geologists based on in situ monitoring of the material and geophysical logging conducted in the well after completion of drilling. 

Filter-point wells

 Shallow tube well constructed by driving  small-diameter, perforated tube with a pointed end into friable ground with gravel aquifers using air or water jetting. A filter, at the lower section helps to filter the sand and other particles and prevent them from penetrating into the well.  Mainly used to draw water from shallow depths. Mostly , found in the coastal stretches . 

Infiltration gallery

Infiltration gallery or Collector well connects with horizontal perforated  pipe surrounded  by gravel filter enveloped  radially in a permeable aquifer with  high water table and a continuous recharge with  perennial flow.  Which usually laid parallel to river beds for intercepting and collecting groundwater by gravity flow. Most of the collector wells in rivers ensure perennial yield and use as drinking water. Used in coastal sandy terrains 

Tunnel wells  

Traditional water management system  to provide  reliable supply of water for human settlements and irrigation in Kasaragod district of Kerala. Horizontal tunnels dug in the slope of  laterite hill for 30 to 40 m , use gravitational force to extract  groundwater and finally store it into a tank. During the 1950s around 5000 Tunnel wells  ,constructed  in Kerala and Karnataka.

The tunnels are generally rectangular or dome-shaped with an optimal height and width which allows a man to work comfortably. It’s made with a downward slope to use the gravitational force for collection of  water percolating outside. During construction, lining in walls  prevents collapse due to loose or soft soil. Air shafts in longer surangams  supply fresh air and expel poisonous gasses. They connect to one another. Water  collected by  temporary small barriers and stored in a storage pit or tank. Then  taken to farms by siphon methods, by creating aqueducts. One of the problems associated with the surangams is water wastage. 

Groundwater Abstraction

 The groundwater abstraction or draft in Kerala is mainly for irrigation and domestic uses.  Kerala has the highest well density among all the states  .

Quality

The usability of groundwater is determined by its quality in terms of physical, chemical and bacteriological aspects.  Quality aspects much more than the quantity, as it affects the usability. The groundwater – clean, colorless and odorless with little or no suspended matter in most hydrogeological situations and therefore  used directly . Some of the physical properties  critical to restrict the usability of groundwater are color, odor and turbidity.

In addition, temperature another parameter that indicates reactive groundwater regime due to natural processes or external interferences. The chemical quality of groundwater  depends on its source, geology, climate and environment .  Another quality index of groundwater – biochemical oxygen demand (BOD). BOD means the amount of oxygen required to cause the biological decomposition of organic matter. Induction of harmful substances from human interactions in the groundwater alters its properties as groundwater pollution. Pollutants such as  physical characters, ionic matter, organic compounds, organic materials, bacteriological matter and radioactivity.

 Physico-Chemical Properties 

The groundwater in Kerala ,mostly acidic in nature and  rest alkaline. Almost 62% of the samples vary from 4.32 to 6.46 (less than the desirable limit of 6.5–8.5). The low pH of groundwater  generally attributes to sulphide oxidation , acidic nature of the soil or aquifer characteristics .We can  improve Ph by adding clam or oyster shells to drinking water. Generally, the salinity of groundwater is within the desirable limit (< 500 ppm) . The important constituents contributing alkalinity are bicarbonate (HCO3 ¯), carbonates (CO3 2¯) and hydroxyl (OH¯) anions (Weiner, 2000). Alkalinity exceeded the desirable limit of 200 mg/L at Plachimada (340 mg/L) and Kollengode (408 mg/L). The strong positive correlation between Ca2+, Mg2+, HCO, alkalinity and hardness indicates that magnesium contributes  alkalinity rather than calcium.

Chloride – desirable limit in almost 97% of the samples studied.

 Calcium concentration – within the desirable limit (75 mg/L) as found in 97% of the samples. 

 Calcium in groundwater is mainly due to the dissolution of minerals and its higher concentration 

 Nitrate –  within the desirable limit of 45 mg/L

The concentration of sulphates is also under the desirable limit (200 mg/L) 

 According to Weiner (2000), high concentration of SO4 2¯ leads to diarrhea. The concentration of fluorides in groundwater exceeded the desirable limit (1 mg/L) 

Management of Groundwater Quality

The physico-chemical parameters of groundwater in Kerala, is not affected by anthropogenic activities,  from  nitrate and sulphate values. However, the groundwater regime of Kerala, especially shallow aquifers, exhibits low pH with fecal coliform bacteria due to unhygienic practices. Therefore,  maintain the pH levels within the desirable limit and curb the bacterial contamination.

The health hazard due to coliform contamination is minimized by maintaining better hygiene with good sanitation .Groundwater quality management ensures availability of colorless ,transparent water without foul smell, bad taste, detrimental chemicals and high acidity or alkalinity. 

Use of Groundwater

The first priority should be that groundwater should meet  the rural and urban population. The primary groundwater uses should also include direct use of groundwater for livelihoods and municipal use. The secondary water uses include commercial activities, including power generation, industry and large scale commercial farms. . 

Pricing for Industrial Use of Groundwater

 The industrial or bulk groundwater use should  price  a water rate, as prescribed by the appropriate authority . In the case of agricultural use, the input subsidies for water should be subject to periodical revision and water efficiency.

Conjunctive use of groundwater and surface water 

 Practice the conjunctive  use of groundwater and surface water to maximize the economic ,environmental effects and optimize the water demand and supply balance . 

 In reality,  conjunctive use can be  practiced widely on a spontaneous basis, in response to inadequate availability of irrigation canal water . This often leads to the use of a large proportion of groundwater even in major irrigation canals. 

 Issues working against the conjunctive use : 

• Socio-political sensitivity and unwillingness of farmers in areas  with surface water to allow canal water to reach less endowed areas
• Inadequate understanding of conjunctive use and the potential role of groundwater by water resource planners, administrators and politicians
•  Inadequate knowledge of the degree to which private groundwater use by farmers  
•  Ineffective implementation of Participatory Irrigation Management (PIM)

 In order to overcome the impediments to conjunctive use, it is necessary to have committed and enforced administration . This requires significant strengthening of the institutional arrangements . The conjunctive use of surface and ground water resources is still in a conceptual stage . The conjunctive use of surface and ground water resources is still in a conceptual stage as far as Kerala is concerned. 

Water !

 “  Save every Drop “