Comparison of indices for scaling and corrosion tendency of groundwater: Case study of unconfined aquifer from Mahoba District, U.P. State

This paper is an attempt to utilize the various indices available to assess the degree of corrosivity and calcite formation of groundwater sampled from Indian Mk. II handpumps from the district of Mahoba in Uttar Pradesh. The indices used include Langelier Saturation Index, Ryznar Stability Index, Puckorius Scaling Index, Larson-Skold Index, and Potential to Promote Galvanic Corrosion. Corrosivity of groundwater would cause materials used in construction of pipes to leach into drinking water since the same is used for household activities and drinking purposes. Calcite formation would affect the amount of exertion used to manually pump out required quantity of groundwater from the aquifer as it reduces the convey potential of pipes. One hundred five groundwater samples were collected from Indian Mk. II handpumps tapping shallow aquifer (up to 35mbgl) from different locations that were used by locals for analysis of basic parameters like pH, TDS, EC, ions like calcium, magnesium, sodium, potassium, chloride, carbonate, bicarbonate, sulphate, etc. Overall, majority of the groundwater samples display tendency to deposit calcium carbonate within the pipes, chlorides and sulphates not interfering with natural film formation in pipes and possibility of galvanic corrosion, whereas minority of the samples indicate the alternate scenario.


Introduction
Incrustation is the process of deposition of precipitate from groundwater onto well installation materials or into the surrounding aquifer. The implications include reduction in open area of well screen in wells and reduction of aquifer permeability adjacent to the well in addition to causing blockages in convey pipes and increase in operational and maintenance costs. Incrustation relates directly to supersaturation of groundwater with respect to certain compounds, like calcite, carbonates, iron compounds, sulphates or manganese compounds. Water being a universal solvent dissolves many constituents from atmosphere, soil and strata in contact. These chemical constituents are directly responsible for causing corrosion, fouling and scaling.
Scaling and fouling indices have been used to predict the extent of calcium carbonate deposition onto heat transfer surfaces in industries. Once calcium carbonate is supersaturated, it precipitates as salt with an increase in temperature. Saturation indices of calcium carbonate have been determined mainly by saturation Indices (Nalco 1979) even though sophisticated methods are available (Hasson 1981).
Some authors have calculated the scale deposition and corrosion in tap waters (Al-Rawajfeh and Al-Shamaileh 2007), whereas others have described the same for river waters (Haritash et al. 2016) and others have described its usefulness in water supply networks and water treatment and supply plants (Davil et al. 2009;Mirzabeygi et al. 2017).

The study area
Mahoba district lies at the southernmost extent of Bundelkhand region of Uttar Pradesh state between 25°01′30″ to 25°39′40″ North latitude and 79°15′00″ to 80°10′30″ East longitude. The district experiences semiarid climate with low precipitation of around 864 mm/year and covers an area

Geology and hydrogeology of study area
The district can be broadly classified into two physiographic units-southern portion with high relief and northern portion with relatively low relief and low hillocks. Geologically, the northern portion of the district comprises Quaternary alluvium underlain by Bundelkhand Granites of Precambrian age. Dolerite dykes and Quartz reefs are also seen at places but form only a fraction when compared to Granites and Alluvium, and they too were formed during Precambrian age. Granites are observed as isolated or clustered hillocks. Granites occurring in the district reflect considerable heterogeneity in colour, texture, grain size and composition. Most common Granite in the district is Alkali feldspar Granite, and less common variety is grey coloured Leucogranite. Occasionally, Quartz veins are also encountered in Granite. The Quartz reef trending NE-SW occurs as narrow ridges. These reefs are composed of fine grained compact silica material and are milky white in colour. Dolerite dykes intruding the Granite mainly display trend in EW to NW-SE direction (CGWB 2014).
Surface water from Madan Sagar, Kirat Sagar, Vijay Sagar and Kalyan Sagar lakes is a possible source of potable water but it is only fit for agriculture (Pal et al. 2013). All villages and towns depend solely upon groundwater for sustenance, since surface water bodies have either disappeared or have been polluted. Groundwater occurs up to 35 mbgl (metres below ground level) in the form of phreatic aquifer and is mainly tapped by dug wells, Indian Mk. II handpumps and shallow borewells for supplying drinking water and irrigation water, wherever possible.
Yield of dug well and shallow borewell ranges from 100 to 300 lpm. Deeper groundwater is tapped by borewells that Fig. 1 Map of the study area access fractures and joints below 35 mbgl. The Granites are profusely and extensively jointed and fractured, forming good conduit for groundwater movement. Groundwater occurs in deeper fracture zones in semi-confined to confined condition. Quartz reefs act as a barrier impeding the subsurface movement of groundwater.
Geological map of Mahoba is attached in Fig. 2.

Materials and methods
The The samples were collected after removal of water column by pumping out the water from the handpumps for at least 5-10 min, and the water samples were collected in HDPE sample bottles of 1 L capacity. It was ensured that the sample bottles were free from air bubbles and they were immediately closed, labelled and transported to a NABL accredited laboratory at Panipat for analysis. Map displaying sampling locations is attached in Fig. 3.

Hydrogeochemistry
The groundwater samples collected were plotted on the Hill-Piper trilinear diagram (Piper 1944), and the samples were classified into the following categories based on the dominant cation and anion.
A closer look at the data reveals Ca> Na> Mg > K in cations and HCO 3 > Cl> SO 4 > CO 3 in anions.
The plotted Hill-Piper trilinear diagrams are attached in Fig. 4.

Calculation of corrosion indices
The following indices were calculated.

Langelier saturation index
Langelier 1936 had proposed a formula to predict calcium carbonate scaling in water based on pH, TDS, temperature, total alkalinity and hardness that affect calcium carbonate solubility and was calculated by the following formula where pH is the actual pH of water and pHs is the pH at saturation and is calculated by pHs = (9.3+A+B)-(C+D).  LSI value > 0 indicates water is supersaturated and CaCO 3 layer precipitates, LSI value = 0 indicates water is in equilibrium with CaCO 3 , and no precipitation or corrosion is possible, LSI value < 0 indicates water is under-saturated, dissolves solid CaCO 3 and causes corrosion.

Puckorius scaling index
Puckorius and Brooke 1991 also proposed a formula that is calculated by the following formula where pH s is the pH at saturation and pH eq is the pH at equilibrium and calculated by pH eq = 1.465 * log 10 (Alkalinity) + 4.54 PSI value < 6 indicates CaCO 3 scale will be dissolved by water and is corrosive, PSI value > 6 indicates CaCO 3 scaling may occur. Ryznar 1949 also proposed a formula to predict calcium carbonate scaling in water that overcame deficiencies in LSI and is calculated by the following formula RSI value < 6 indicates increasing tendency for scaling with decreasing index, RSI value between 6-7 indicates formation of no corrosion-protective film by water, RSI value > 8 indicates tendency for corrosion by water.

Larson-Skold index
Larson and Skold 1958 gave a formula to predict the corrosivity of water towards mild steel which is calculated by the following formula All ionic concentrations in meq/l. LS value < 0.8 indicates chloride and sulphate may not interfere with natural scale formation, LS value between 0.8 to 1.2 indicates chloride and sulphate may interfere with scale formation and higher than desired corrosion rates expected, LS value > 1.2 indicate very high corrosion rate by the action of chloride and sulphate. Edwards et al. (2007) proposed a formula to calculate possibility of release of lead from copper pipes wherein lead was used as solder PPGC > 0.50 indicates susceptibility of galvanic corrosion by groundwater.

Potential to Promote Galvanic Corrosion (PPGC)
All ionic concentrations are in mg/l. Computed data of corrosion indices is attached in Appendix 2.

Results and discussion
( 13.3% of samples indicate tendency for high corrosion.
13.33% of samples indicate higher than desired corrosion rates wherein chlorides and sulphates may interfere with natural film formation.
The groundwater samples mainly obtained from Precambrian Granites display possibility of corrosion with reference to Langelier Saturation Index, Puckorius Scaling Index, Ryznar Stability Index and Larson-Skold Index, whereas very few groundwater samples collected from Quartz reefs and Quaternary alluvium mirror the possibility of corrosion with respect to indices stated above.
It is also observed that the underlying geology has little to no bearing on corrosion with respect to PPGC (index that indicates possibility of galvanic corrosion) and about 94.28% of samples display possibility of galvanic corrosion across varied lithologies barring a few samples that possess low sulphate content.
Overall, majority of the groundwater samples display tendency to deposit calcium carbonate within the pipes, chlorides and sulphates not interfering with natural film formation in pipes and possibility of galvanic corrosion, whereas minority of samples indicate the alternate scenario.
The map of GW samples classified with respect to different corrosion indices is attached in Fig. 5.

Conclusion
It is observed that groundwater samples collected from Precambrian Bundelkhand Granites display tendency to deposit calcium carbonate as per 3 indices-Langelier Saturation Index, Puckorius Scaling Index and Ryznar Stability Index in addition to possibility of corrosion of mild steel as defined by Larson-Skold Index.
The geology of the study area was found to have none to near negligible influence on Potential to Promote Galvanic Corrosion (PPGC) index as the quantum of sulphate ion varies across different lithologies and no relation between the two is observed.
The various indices can be utilized to plan for materials used in construction of Indian Mk. II handpumps in addition to deciding the material used for borewell casing that is resistant to corrosion as calculated by different indices. The groundwater samples will require collection every month or every quarter in order to calculate values as defined by each index. Galvanic pipes containing lead as solder should be avoided since the index indicates high possibility of leaching of lead into water.
The data computed from various corrosion indices can also be utilized prior to establishment of industries that require stringent water quality standards for production, and areas free from corrosive potential of groundwater can be demarcated for investment in infrastructure in the future. Fig. 4 a-