OR/14/068 Methodology

Lapworth, D J, Gopal, K, Rao, M S, and MacDonald, A M. 2014. Intensive groundwater exploitation in the Punjab — an evaluation of resource and quality trends. British Geological Survey Internal Report, OR/14/068.

As part of this case study, historical groundwater level data (1975–2012) for the Bist-Doab was obtained from the CGWB groundwater monitoring database. Sites with groundwater level data in Bist-Doab are shown in Figure 4, some of the large urban centres in the catchment are also shown for reference. A total of 123 sites (yellow sites in Figure 4) had water level records, however only 43 (green sites in Figure 4) of these had long-term records, i.e. >20 years, with useful frequency. Most long-term monitoring sites have 4 water level records each year, taken as manual dips in May, August November and January, to capture the pre-Monsoon and post Monsoon changes in groundwater level across the catchment.

Figure 4 CGWB groundwater level records in Bist-Doab, grey squares show selected urban centres (source: CGWB).

A preliminary analysis of three key components of the groundwater level data was undertaken for the long-term monitoring sites:

i) the degree of groundwater level decline

ii) the minimum groundwater level under pre monsoon conditions and

iii) the nature of long-term post monsoon recovery in groundwater level

The level of decline, pre monsoon depth and recovery during monsoon were mapped across the catchment to investigate spatial variations in groundwater resources and long-term security for shallow abstraction.

Environmental tracers

Chemical properties of groundwater can be used as environmental tracers and so enable conclusions to be drawn about the water’s origin, residence time and hydrogeochemical evolution. In addition to stable isotopes (δ¹⁸O and δ²H) major elements (e.g. NO₃, Mg/Ca) and trace elements (e.g. Sr, Li, Rb, Mo), which are common in many hydrogeological investigations (e.g. Edmunds et al., 2003), two specialised tracer techniques have been used in this study: Chlorofluorocarbons (CFCs) and SF₆ trace gases. Additional samples for noble gas analysis were also taken to obtain recharge temperatures, these results will not be discussed in this report as analysis is as yet incomplete.

Stable Isotopes

Stable O and H isotopes are tracers of physical processes that water molecules undergo between evaporation from the ocean and arrival in the aquifer via recharge of rainfall (Clark and Fritz 1997) They are typically used in semi-arid hydrogeological studies to indicate the degree to which waters may have been modified prior to recharge or the existence of pre-Holocene waters. In addition, stable isotopes and other conservative anions such as Cl and Br can be used as field tracers when applied to the surface. A small pilot study using this method to estimate recharge velocities in the shallow unsaturated zone was carried out as part of this study, however the analysis for these samples is not complete and the results from this work will not be covered in this report.

Trace gas age indicators

The use of CFCs and SF₆ as groundwater age tracers relies on the rise in their atmospheric concentrations over the last 50 years together with certain assumptions about atmospheric mixing and recharge solubility (Plummer and Busenberg 1999). These gases are known to be well-mixed in the atmosphere so the curves are considered to be applicable to the study area. The use of several trace gases is recommended as under certain conditions individual tracers may have limitations (Darling et al. 2012) In particular, the CFCs may be affected by pollution, and/or degradation under anaerobic conditions (Plummer and Busenberg 1999), and there are also issues with the use of SF₆ due to terrigenic production (Koh et al. 2007).

Interpreting trace gas indicators relies on consideration of mean recharge temperature, altitude and incorporation of excess air. An average annual air temperature of 26°C was used for this study to represent recharge temperatures. The phenomenon of ‘excess air’ incorporated during recharge has only a small effect on the CFCs but requires correction for SF₆ measurement. Significant numbers of deeper sites in the pre-monsoon sampling round showed enriched SF₆ concentrations, greater than 10 times modern concentrations, suggesting terragenic sources of SF₆ in groundwaters. In addition, due to the elevated temperatures in the region (45–50°C) during the sampling, a number of SF₆ samples expanded and broke the glass bottles and/or lids rendering the samples unsuitable for dating. In light of both of these factors only the CFC data is presented in this report.

Lumped parameter models (LPM) typically used to describe some of the variation seen in groundwater mixtures include piston flow (PFM), exponential mixing (EMM) and binary mixing (BMM) (Zuber 1986; Cook and Böhlke 2000. With the absence of SF₆ data with which to assess groundwater flow processes, for this report two simple mixing models have been used to compare results from different sites across the catchment: i) a binary mixing model (assuming mixing between modern and CFC dead water) to estimate fraction of modern recharge and ii) a simple PFM has been used to estimate mean residence times (MRT) of groundwater samples.

Groundwater sampling

Shallow (approx. <50 m) and a deep (generally >80 m) groundwater was sampled from 19 paired sites (see Figure 5 and Table 2) across the catchment. These were selected to ensure a good geographical spread across the catchment, and to ensure that the three main hydrological setting in the catchment were coved adequately, namely i) the upper NE portion of the catchment within the lower Shiwalik range ii) the centre of the Doab including areas with known groundwater depletion iii) locations in close proximity to both the R Beas and Satluj and at the lower end of the catchment near the confluence of the two Rivers. Only sites which had good borehole completion were selected to minimise localised sources of contamination, and the distance between the shallow and deep sites at each location was minimised. Shallow sites were <50 mbgl, with the exception of one site at Ajnoha, and deep sites were >100 mbgl. Although there were differences in completion depths between locations and shallow sites were all completed in the first sedimentary aquifer, and where records were available, deep sites were shown to be completed in the second or third aquifer. This term ‘aquifer’ refers to a continuous layer of permeable sediment >10 m thick. The aquifers are separated by low permeability clay, silt and kankar deposits.

Groundwater was sampled using existing hand pumps and tube wells during February and May, 2013 for pre-monsoon season and during October, 2013 for post monsoon sampling. The sample locations were recorded using Global Positioning System (GPS). Prior to sampling, boreholes were purged (minimum 3 borehole volumes) to ensure a fresh sample was collected. Groundwater chemistry was monitored carefully for a range of field parameters including electrical conductivity (EC), pH, redox potential (Eh), dissolved oxygen and temperature using a flow-through cell. Only after stable field readings were obtained were samples collected. Field alkalinity was determined by titration in the field using 50 ml sample and 1.6 N sulphuric acid.

Figure 5 Location of paired shallow and deep monitoring sites across the Bist-Doab.

Table 2    Sampling site names, locations, districts and completion details

Site Name	District	Longitude (E)	Latitude (N)	Depth (m) Shallow	Depth (m) Deep
Banga	Nawanshahr	75.5°9'36.4"	31°10'04.4"	16	100
Mehli	Nawanshahr	75.4°8'51.4"	31°12'47.6"	40	150
Phillaur	Jalandhar	75.4°47'26.2"	31°01'24.1"	30	80
Malikpur	Phagwara	75.4°50'07.5"	31°16'55.6"	25	160
Nawanshahr	Nawanshahr	76°07'11.5"	31°07'33.1"	30	130
Maili	Hoshiarpur	76°04'12.7"	31°24'07.3"	45	80
Hariana	Hoshiarpur	76°50'29.6"	31°38'06.2"	50	160
Aima Mangat	Hoshiarpur	75°37'57.6"	31°53'32.5"	20	85
Arjanwal	Jalandhar	75°41'44.2"	31°25'13.5"	10	140
Jandiala	Jalandhar	75°37'07.8"	31°09'46.4"	30	60
Saidpur	Jalandhar	75°19'43"	31°05'06.1"	35	122
Mallian K	Jalandhar	75°24'57.1"	31°10'57.4"	35	130
Busowal	Kapurthala	75°09'17.5"	31°12'49.2"	9	130
Boot	Kapurthala	75°23'52.1"	31°27'11.6"	10	130
Garhsankar	Hoshiarpur	76°08'07.8"	31°13'34"	18.3	45.7
Hoshiarpur	Hoshiarpur	75°55'09"	31°31'55.4"	45.7	64
Ajnoha	Hoshiarpur	75°53'46.4"	31°19'36"	67	121.9
Nussi Pind	Jalandhar	75°33'2.5"	31°24'02"	21.3	152.4
Amritpur	Kapurthala	75°10'21.8"	31°22'49.5"	7.6	76.2

Filtered (0.45 μm, cellulose nitrate) water samples were collected in pre-washed plastic bottles. The un-acidified sampling bottles were carefully filled just to overflowing to ensure no air bubble was trapped inside the sample container. The samples were labelled, brought to the laboratory and stored at 4°C to avoid any major chemical alteration prior to analysis. Samples for cation analysis were acidified (1% v/v Aristar nitric acid) on return to the UK prior to analysis. Dissolved organic carbon (DOC) samples were filtered (0.45 μm) in the field using silver filters and were stored refrigerated in glass bottles prior to analysis. At each site samples were also collected for CFC-11 and CFC-12 analysis. CFC and SF₆ samples were collected unfiltered and without atmospheric contact in sealed containers by the displacement method of Oster (1994) This method ensures that the sample is protected from possible atmospheric contamination by a protective jacket of the same water. Stable isotope samples were collected in Nalgene bottles.

Sample analysis

Un-acidified sub-samples were analysed for major anions using ion chromatography. Major and trace cations were analysed by ICP-MS. Stable isotope analysis (δ¹⁸O, δ²H) was carried out using standard preparation techniques followed by isotope ratio measurement on a VG-Micromass Optima mass spectrometer. Data considered in this paper are expressed in ‰ with respect to Vienna Standard Mean Ocean Water (VSMOW). CFCs and SF₆ were measured by gas chromatography with an electron capture detector after pre-concentration by cryogenic methods, based on the methods of Busenberg and Plummer (1999). Measurement precision was within ± 0.1‰ for δ¹⁸O and ± 1‰ for δ²H, and ±5% for the CFCs, with detection limits of 0.01 pmol/L (CFC-12), 0.05 pmol/L (CFC-11) and 0.1 fmol/L (SF₆). Measurement of inorganic chemistry, DOC, stable isotopes values, CFCs took place at BGS laboratories in the UK.

Installation of water level and sec loggers

To record daily and seasonal fluctuations and abstraction effects on groundwater 6 piezometers (at depth of 150 mbgl) at Bhogpur, Kapurthala, Nakodar, Saroya, Sultanpur Lodhi and Tanda were drilled and instrumented with automatic water level recorders (Figure 6). Water levels in paired shallow piezometers are also monitoring to investigate interactions between shallow and deep aquifers. Four conductivity loggers are installed in shallow monitoring piezometers (60 mbgl) developed by Punjab Water Resources and Environment Directorate, Chandigarh, at sites Saroya, Bhogpur, Kapurthala and Sultanpur Lodhi (Figure 4). Preliminary results from the data loggers are shown in Appendix 2, Figure 2 as holding data, a full download will be carried out in September after a full hydrological year.

Figure 6 Locations of paired shallow and deep piezometers where continuous loggers are installed to monitor water levels and SEC (shallow sites).