Groundwater quality beneath an Asian megacity on a delta: Kolkata s (Calcutta s) disappearing arsenic and present manganese.

Transcription

1 SUPPORTING INFORMATION Groundwater quality beneath an Asian megacity on a delta: Kolkata s (Calcutta s) disappearing arsenic and present manganese. J.M. McArthur * P.K. Sikdar, M. Leng, U. Ghosal, and I. Sen. Earth Sciences, University College London, Gower Street, London WC1E 6BT *j.mcarthur@ucl.ac.uk, corresponding author. Department of Environment Management, Indian Institute of Social Welfare and Business Management, Management House, College square (West), Kolkata p_sikdar@hotmail.com M.J. Leng, NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK and Centre for Environmental Geochemistry, School of Biosciences, Sutton Bonington Campus, University of Nottingham, Loughborough, LE12 5RD, UK. mjl@bgs.ac.uk S1. Methods Water Analysis: Both public wells (Fig. S1 left) and municipal wells (Fig. S1 right) were sampled after purging. Samples were filtered through 0.45 micron filters only when perceptibly cloudy. Samples for analysis of cations, total-p (reported as PO4), B, Br, and trace elements including As, were acidified in the field to be 1% with respect to nitric acid and diluted 1:1 with 1% HNO3 before analysis. Samples for F, Cl, SO4 and NO3 analysis were not acidified. Samples were analysed by ICP-AES for major elements and P using matrix-matched standards with appropriate dilution of samples to minimise matrix effects. Samples were analysed by ICP-MS for As, Br, and other trace elements using matrix-matched standards and appropriate dilution. Total Br was analysed separately with separate matrix-matching after dilution to bring Br concentrations into the range 0 50 ppb and with washout times long enough to eliminate memory effects during analysis. Samples were analysed for F, Cl. NO3 and SO4 using ion chromatography with dilution where appropriate. Oxygen isotope (δ 18 O) measurements were made using the CO2 equilibration method with an Isoprime 100 mass spectrometer with an Aquaprep device. Deuterium isotope (δd) measurements were made using an online Cr reduction method with a EuroPyrOH-3110 system coupled to a Micromass Isoprime mass spectrometer. Isotope measurements used internal standards calibrated against the international standards VSMOW2 and VSLAP2. Errors are typically ± 0.05 for δ 18 O and ±1.0 for δd. Lithology: the spatial distribution of the sediments underlying the area in and around Kolkata city has been ascertained from lithlologs of 109 boreholes (Fig. 1) non-uniformly distributed. The boreholes were drilled by the Geological Survey of India, Central Ground Water Board, State Water Investigation Department (West Bengal), Kolkata Municipal Corporation and private drilling agency like Pioneer Tubewell Industries Private Limited during the period (Sikdar, 1996). The thickness and extent of each litho-unit are inhomogeneous at subsurface. 1

2 The subsurface geology of Kolkata (Fig. 4) is constructed using the solid modelling concept in RockWorks 15. The closest-point gridding method generated a solid model from the irregular distribution of the boreholes. Each grid node was assigned a value equal to the value at the closest borehole. The model thus generated is of regularly spaced nodes from irregularly spaced data by interpolating measured values of lithology. The model resolution is 1850 m (X) x 1800 m (Y) x 250 m (Z) and the resulting discretization is 38 X nodes x 37 Y nodes x 26 Z nodes. Fig. S1. Wells in Kolkata: left, hand-pumped public well; right, electrically-pumped, high-capacity, municipal well. 2

3 S2. Sulphate Sulphate is removed from groundwater by reduction to FeS2 and H2S, and added to groundwater by salt-water mixing and by addition of SO4-rich waste water. Compositions would plot along the marine mixing line were reduction and waste water not important. Fig. S2a. Cross-plot of SO4 and Cl. Reduction of SO4 causes most samples to plot below the marine mixing-line, except at low chlorinities, where excess SO4 is derived from sewage effluent and causes the points to plot above the line S2b. Map of excess SO4 in groundwater. Excess calculated as measured SO4 expressed as a percentage of marine-sourced SO4. High concentrations are found both around and north of the ACPL site and along Tolly s Nala, and in the region 1 km south of the ACPL site where isotopically-light water from Tolly s Nala is infiltrating to the late Pleistocene aquifer. 3

4 S3. Health Hazards from other Trace and Minor Elements Arsenic: concentrations of As are > 10 g/l in groundwater from 5% of wells (6% public, 1 % municipal - one well, No 33). Public well (No. 71), with 69 g/l As, is the only well of 280 sampled that gave groundwater exceeding the drinking water standard for India of 50 g/l. Bromide: Concentrations range up to 5990 g/l and in themselves pose no risk to health. Such high concentrations of Br (and Cl) give rise to a risk of forming chlorinated and brominated disinfection by-products on treatment of the groundwater, and a risk of forming bromate if ozonation is practiced as a disinfection method. Fluoride: no concentration of F exceed 1.5 mg/l, the WHO (2017) GV for F 67. Local adjustments are recommended where water intake is high, as it is in hot countries, so we take a value of < 0.75 mg/l. No groundwater from a municipal wells exceeds 0.75 mg/l F. Groundwater from 10 public wells (3%) exceed 0.75 mg/l with a maximum of 1.0 mg/l. Iodine: some 40% of public wells and 20% of municipal wells give groundwaters having concentrations of total-i (iodine) > 40 g/l, a concentration at which I in drinking water begins to make a serious contribution to the recommended daily intake of iodine for humans of μg/day (depending on maturity and gender 78 ). Sodium. The WHO (2017) Guidelines for Drinking Water Quality 67 state that the average taste threshold for sodium is about 200 mg/l. No health-based guideline value has been derived (see sections and 12.1), as the contribution from drinking-water to daily intake is small. Whilst intake from drinking water may be small in developed countries, many coastal and deltaic groundwater in less developed regions have concentrations of Na in drinking water that is above 200 mg/l, the threshold for taste. Concentrations of Na in municipal wells of Kolkata are mostly in the low hundreds (median 281 mg/l) but range up to 737 mg/l. Concentrations in public wells are lower (median 135) but range up to 529 mg/l. Concentrations of Na exceed 500 mg/l in 14% of groundwaters from municipal wells and in one public well. Consumption of such groundwater may contribute to adverse health effects 69,70 by enhancing the daily intake of Na to levels above 2,000 mg/day. 68 Other species. For B, Ba, Cd, Cu, Cr, Ni, Sb, Se, U, concentrations are below, and typically well below, the WHO GVs for drinking water. For Mo, Bi, V, no GV s are set by WHO 67 ; concentrations are < 3 g/l. Nitrate and Nitrite: groundwaters contain 3.2 mg/l NO3 and so are below the WHO GV for 2017 of 50 mg/l. Nitrite was not detected in any groundwater. Groundwater from Well 33 contained the maximum concentration of NO3 of 3.2 mg/l; no other groundwater exceeded 1.8 mg/l and most had undetectable NO3. Lead: no groundwaters from municipal wells exceed the WHO GV of 10 g/l 67 and only three exceed 1 g/l with a maximum of 5.6 g/l. In public wells, 5% of groundwaters (11 wells) exceed the WHO GV for Pb, with a maximum of 44 g/l. The contamination is not in the aquifer because 4

5 Pb is too particle reactive to stay in solution at the circum-neutral ph of the groundwaters. The contamination probably comes off pump fittings. Overall quality. If all potential hazards are compounded, then 64% of public wells and 40% of municipal wells are supplying groundwater unfit for use because of the presence of one or more of the contaminant/pollutant species As, F, Mn, Na, or Pb in excessive amounts. S3. Iron and Manganese Overall, 45% of wells sampled in Kolkata gave groundwater containing > 0.4 mg/l of Mn. One municipal well, No 33, is anomalous in being the only one with Mn concentrations > 1 mg/l, and then only when dilution from river water is at a minimum (February, 2016). In Fig. S2 we show a map of Mn concentrations > 1 mg/l. Concentrations of Fe do not exceed 5 mg/l in any municipal well. In public wells, concentrations reach 22 mg/l and are mostly located near the ACPL site and/or Tolly s Nala Fig. S3. Map of Mn and Fe concentrations in groundwater. Squares are public wells,, circles are municipal wells. Open symbols are wells with Mn < 1 mg/l and Fe < 10 mg/l. Where groundwaters have either > 1 mg/l Mn or > 10 mg/l Fe, the symbol is black-dotted. 5

6 S4. Fluoride concentrations No concentration of F exceeds 1 mg/l and concentrations are inversely related to Ca concentrations, a relation seen in many groundwaters and attributed to equilibrium with insoluble Ca phases, notably apatite and/or fluorite. Fig. S4. Cross-plot of F against Ca, showing control of F concentration by fluorite or apatite. Symbols as in Figs. 2a, 5a. S5. Mixing and Ion-exchange. The main control on major element composition of KMC groundwater is mixing with salt-water and ion-exchange. The latter is illustrated by Fig. S5a, which shows salt-corrected Na varying inversely with salt-corrected Ca. Concentrations of Na range around zero because ion-exchange is the only process governing Na concentrations once the salt-correction removes the marine-mixing influence Fig. S5a. Salt-corrected Ca v. salt-corrected Na. 6

7 For Ca, calcite dissolution and feldspar weathering also contribute to Ca, so and no simple correction exists for these processes and Ca does not range about zero. Where a deficit of Na is seen, the groundwaters have lost Na to mineral surfaces in exchange for Ca; this process occurs when sea water, or brackish water of marine origin, invades and aquifer. Where and excess of Na is seen, Ca-rich fresh water is invading the aquifer and losing Ca to mineral surfaces in exchange for Na, which is therefore in excess in solution. Note that high Na does not unequivocally diagnose saline intrusion. For KMC groundwater, the flushing has elevated Na concentrations in groundwater under south-western KMC, whilst saline intrusion characterises groundwater under north-eastern KMC (Fig S5b) Fig. S5b. Distribution of Excess Na from ion-exchange in KMC grounwater. 7

8 Table S1. Well parameters and compositional data for groundwaters sampled in the area of the Kolkata Municipal Corporation. Well No. Latitude Longitude Diameter Depth Power Date Ward Borough Date δ 18 O δd Na K Ca Mg Sr Ba Fe Mn F Cl Br I NO3 SO4 HCO3 PO4 H 4SiO 4 B V Cr Co Ni Cu As Se Mo Cd Sb Pb Bi U metres HP Installed Sampled V SM OW 2 V SM OW 2 mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l g/l g/l mg/l mg/l mg/l mg/l mg/l g/l g/l g/l g/l g/l g/l g/l g/l g/l g/l g/l g/l g/l g/l Municipal Wells, 1 to KOL "/8" IX KOL "/8" XV KOL "/8" XV KOL "/8" III KOL "/8" III KOL "/8" III KOL "/8" VI KOL "/8" VI KOL "/8" VI KOL "/8" VI KOL "/8" VI KOL "/8" VII KOL "/8" VII KOL "/8" VII KOL "/8" VII KOL "/8" VII KOL "/8" III KOL "/8" III KOL "/8" III KOL "/8" III KOL "/8" III KOL "/8" III KOL "/8" VII KOL "/8" XII KOL "/8" XII KOL "/8" XII KOL "/8" XII KOL "/8" XII KOL "/8" XII KOL "/8" X KOL "/8" X KOL "/8" X KOL "/8" X KOL 33 Rpt "/8" X KOL 33 Rpt "/8" X KOL 33 Rpt "/8" X KOL "/8" VIII KOL "/8" VIII KOL "/8" XII KOL "/8" XII KOL "/8" X KOL "/8" VIII KOL "/8" IX KOL "/8" XI KOL "/8" X KOL "/8" VIII KOL "/8" X KOL "/8" X KOL "/8" X KOL "/8" X KOL "/8" XIII KOL "/8" XI KOL "/8" X KOL "/8" XIII KOL "/8" XIII KOL "/8" XIII KOL "/8" X KOL "/8" XI KOL "/8" XI KOL "/8" XI KOL "/8" XI KOL "/8" X KOL "/8" XI KOL "/8" XI KOL "/8" XI KOL "/8" XI KOL "/8" X KOL "/8" X KOL "/8" X KOL "/8" XIV KOL "/8" XVI KOL "/8" XIV KOL "/8" XIV

9 Table 1 ctd. Well No. Latitude Longitude Diameter Depth Power Date Ward Borough Date δ 18 O δd Na K Ca Mg Sr Ba Fe Mn F Cl Br I NO3 SO4 HCO3 PO4 H 4SiO 4 B V Cr Co Ni Cu As Se Mo Cd Sb Pb Bi U metres HP Installed Sampled V SM OW 2 V SM OW 2 mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l g/l g/l mg/l mg/l mg/l mg/l mg/l g/l g/l g/l g/l g/l g/l g/l g/l g/l g/l g/l g/l g/l g/l Public Wells, 71 to 280 KOL 71(repeat) Hand Pump 94 X KOL Hand Pump 94 X KOL Hand Pump 111 XI KOL Hand Pump 100 X KOL Hand Pump 100 X KOL Hand Pump 102 XII KOL Hand Pump 96 X KOL Hand Pump 93 X KOL Hand Pump 90 VIII KOL Hand Pump 69 VIII KOL Hand Pump 91 X KOL Hand Pump 32 III KOL Hand Pump 32 III KOL Hand Pump 81 X KOL Hand Pump 113 XI KOL Hand Pump 114 XI KOL Hand Pump 115 XIII KOL Hand Pump 115 XIII KOL Hand Pump 115 XIII KOL Hand Pump 115 XIII KOL Hand Pump 115 XIII KOL Hand Pump 98 X KOL Hand Pump 98 X KOL Hand Pump 95 X KOL Hand Pump 97 X KOL Hand Pump 115 XIII KOL Hand Pump 115 XIII KoL Hand Pump 94 X KOL Hand Pump 94 X KOL Hand Pump 94 X KOL Hand Pump 94 X KOL Hand Pump 94 X KOL Hand Pump 94 X KOL Hand Pump 94 X KOL Hand Pump 94 X KOL Hand Pump 94 X KOL Hand Pump 94 X KOL Hand Pump 94 X KOL Hand Pump 94 X KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII

10 Table 1 ctd. Well No. Latitude Longitude Diameter Depth Power Date Ward Borough Date δ 18 O δd Na K Ca Mg Sr Ba Fe Mn F Cl Br I NO3 SO4 HCO3 PO4 H4SiO4 B V Cr Co Ni Cu As Se Mo Cd Sb Pb Bi U metres HP Installed Sampled V SM OW 2 V SM OW 2 mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l g/l g/l mg/l mg/l mg/l mg/l mg/l g/l g/l g/l g/l g/l g/l g/l g/l g/l g/l g/l g/l g/l g/l Public Wells, 71 to 280 KOL Hand Pump 118 XIII KOL Hand Pump 118 XIII KOL Motorpump 94 X KOL Motorpump 94 X KOL Motorpump 94 X KOL Motorpump 94 X KOL Hand Pump 3 I KOL Hand Pump 3 I KOL Hand Pump 5 I KOL Hand Pump 5 I KOL Hand Pump 11 II KOL Hand Pump 11 II KOL Hand Pump 12 II KOL Hand Pump 12 II KOL Hand Pump 25 IV KOL Hand Pump 27 IV KOL Hand Pump 26 IV KOL Hand Pump 17 II KOL Hand Pump 10 II KOL Hand Pump 17 II KOL Hand Pump 16 II KOL Hand Pump 32 III KOL Hand Pump 32 III KOL Hand Pump 46 VI KOL Hand Pump 46 VI KOL Hand Pump 47 VI KOL Hand Pump 39 IV KOL Hand Pump 43 V KOL Hand Pump 75 IX KOL Hand Pump 75 IX KOL Hand Pump 75 IX KOL Hand Pump 75 IX KOL Hand Pump 77 IX KOL Hand Pump 77 IX KOL Hand Pump 77 IX KOL Hand Pump 82 IX KOL Hand Pump 71 IX KOL Hand Pump 70 VIII KOL Hand Pump 7 I KOL Hand Pump 8 I KOL Hand Pump 9 I KOL Hand Pump 19 II KOL Hand Pump 30 III KOL Hand Pump 30 III KOL Hand Pump 33 III KOL Hand Pump 35 III KOL Hand Pump 35 III KOL Hand Pump 56 VII KOL Hand Pump 131 XIV KOL Hand Pump 131 XIV KOL Hand Pump 131 XIV KOL Hand Pump 131 XIV KOL Hand Pump 131 XIV KOL Hand Pump 129 XIV KOL Hand Pump 129 XIV KOL Hand Pump 129 XIV KOL Hand Pump 129 XIV KOL Hand Pump 127 XIV KOL Hand Pump 127 XIV KOL Hand Pump 127 XIV KOL Hand Pump 127 XIV KOL Hand Pump 127 XIV KOL Hand Pump 127 XIV KOL Hand Pump 56 VII KOL Hand Pump 64 VII KOL Hand Pump 62 VII KOL Hand Pump 63 VII KOL Hand Pump 63 VII