AR is an Academic Editor for PLOS Water. The other authors declare that they have no competing interests towards this manuscript.
River Ganges (locally called as river Ganga) is one of the most scared rivers in India. The river is symbol of hope, faith and is worshipped for its wholesomeness due to its purity and sanctity. Pollution of river water due to anthropogenic activity is a very common issue worldwide. Similarly, river Ganga pollution in India throughout its entire courses, is a major concern due to city outfalls. This river, also named as river Hooghly in West Bengal, India, is exposed to outfalls carrying domestic wastewater of its both bank and their distribution in river Ganga is strongly influenced by season and tide. This study aimed to generate an idea of distance and direction wise changes of concentration of pollutants in wastewater in river Ganga. During 2014, the selection of five major outfalls was done by considering Dissolved Oxygen (DO), Biochemical Oxygen Demand (BOD), heavy metals, total fecal coliform level, and the study continued for next four consecutive years to find out the influence of tide and season. Geographical Information System (GIS) based maps provided a better reflection of these changes. Student’s t-test highlighted the significant changes in concentration of parameters season wise. A significant higher value of DO, BOD, nitrate nitrogen, and chloride were found in pre-monsoon season compared to monsoon season. Regression Equation generated for highly correlated parameters (coliform and heavy metals) helped to predict the level of one parameter with others. The zone of influence of BOD, DO, phosphorus and nitrate nitrogen from each of the five selected outfalls was very prominent. Acoustic Doppler current profiler at two of the five outfalls helped to estimate strip-wise depth average discharge which helped to estimate the value of water quality parameters by Plug Flow Model during high tide and low tide. A strong tidal variation was observed during low tide. This study helped to predict the influential zone from outfalls which will help to generate an alternative solution of river water use. This approach can be applied globally to prepare river water usage guidelines.
River Ganges (locally called as river Ganga), one of the most sacred rivers in India, travels through West Bengal spanning around 570 km. Southern part of the state are deltaic zones called Sundarbans. Tidal nature of the river influences countryside discharge draining into the Bay of Bengal [
The characteristics of river water also change seasonally. Pre-monsoon, monsoon, and post-monsoon seasons has their own influence on quality of river water. Maximum amount of rainfall received in West Bengal during June-September (the range of mean monsoon rainfall) is around 887.9 mm– 2932.6mm [
A significant difference in heavy metal concentrations like Molybdenum (Mo), Selenium (Se), Ni, Cu, Mn, Zinc (Zn), Cd, Fe, and Cr and variation of other water quality parameters like Total Dissolved Solids (TDS), Total Suspended Solid (TSS), Dissolved Oxygen (DO), Biochemical Oxygen Demand (BOD) and pH was observed in river Ganga in Kanpur, Uttar Pradesh. The concentration of heavy metals was observed to be in an increasing trend in summer followed by winter and monsoon. The dissipation rate of heavy metals may be high due to high temperature and low river flow [
Geographical Information System (GIS) mapping always provides a better reflection on changes of water quality parameters in river. Spatial analysis of river water quality using GIS is a very common practice. Worldwide acceptance of GIS tool helps to develop a reliable water quality model. The incorporation of computer hardware, software along with managing, analyzing, and displaying all form of geographically referenced data can be done using GIS. It is a computing platform to represent data on digital map [
To identify outfalls carrying wastewater in between around 25 km of the selected stretch of the river Ganga in West Bengal, starting from downstream at Howrah Station to upstream at Khardah, 24 Parganas (North), was selected as study area. The area selected for the entire study are located in southern part of the Bengal Basin which are covered by quaternary sediments. The eastern part of the Indian subcontinent i.e., Bengal Basin constitutes the biggest fluvial deltaic to the shallow marine sedimentary basin. The basin is comprised of the riverine channel, floodplain, and delta plain environments [
The importance of the study area lied mainly on its soil characteristics, vegetation, relief, and land use. Thick alluviums to flood plain deposits are very significant characteristics of this region. Flood plains are made up of silty sediments near the riverbank and mixed sediments away from the river in the monsoon season. Elevation of the area varies from 1.5 to 15 m. Quaternary deposits are represented by present-day flood plain deposits, older flood plain deposits, and older alluviums over a mantle of lower Quaternary age. Topographically the area is gentle to moderately sloping. A thick alluvial deposit merges with the tidal flat in the southern part. The slope of the area ranges from 2–3 degrees generally but few places have a maximum slope of 5 to 6 degrees. The texture of soil is an important factor for assessing the soil physical environment and is directly interrelated to soil properties like structure, porosity, adhesion, and consistency [
The objective of this study was to find out the influence of tides and seasons on distribution of pollutants in form of wastewater from city outfalls at the selected stretch of river Ganga in West Bengal, India considering river flow and tidal dynamics.
The collection of river water and analysis of all water quality parameters were performed by following standard methods (Table A in
In 2014, a wide range of field survey using mechanized boat helped to identify twenty selected outfalls situated along the bank of river Ganga from Howrah Station to Khardah. Five major outfalls i.e., Circular Canal in Bagbazar, Ghusuri in Howrah, Dakshineswar Canal (adjacent to Border Security Force facility) in 24 Parganas (North), Ballykhal in Howrah, and Khardah Khal (also known as Titagarh Khal) in 24 Parganas (North) considering right and left bank of the river were selected for the entire study on the basis of important water quality parameters like pH, temperature, DO, conductivity, BOD, total hardness, nitrate nitrogen, chloride, and phosphorus and heavy metals [Lead (Pb), Mercury (Hg) and Total Arsenic (As)], total coliform and fecal coliform from outfalls for its seasonal (2015–2018) and tidal variation study (2016–2018) (Figs 2 and 3 in
(Base map was prepared from Landsat 7 satellite image. Local study area boundaries were digitized from toposheets. Satellite images were downloaded from
Wastewater samples were collected at the mouth of outfalls (2 cm depth) during low tide in the month of March-April (Pre-monsoon), August-September (Monsoon) and November-December (Post-monsoon) at fifteen days of intervals at the same location for four times in each season during the 2015–2018 study period. Same pattern was followed for tidal variation study during the 2016–2018 study period.
To obtain statistical significance of seasonal variations among the concentration of parameters, student t-test was performed (N = 16). Mean value of each parameter for each season underwent this independent statistical test. Pre-monsoon data was compared to post-monsoon and monsoon data separately to assess seasonal variation statistically. p-value indicates the probability of obtaining results, when ≤ 0.05 was considered as significant [
The prediction of water quality parameters was performed by regression analysis. Cumulative data of water quality parameter from 2015–2018 at each sampling site was considered for statistical analysis. A total of 48 samples (N = 48) for each parameter at every sampling site were used to generated correlation matrix. The positively correlated parameters having greater than 0.9 correlation coefficients (r) value was considered to generate regression equations. Among them, the parameters having the value of R2> 0.9 was taken into consideration to predict values [
The influence of tide on selected outfalls of river Ganga was observed seasonally by analyzing river water from different directions from outfalls during 2016–2018. For tidal variation study, water samples were collected across the river (from the outfall), diagonally (from the outfall) and along the bank (in favour of the tidal directions) of river Ganga considering the position of each outfall (
Satellite image followed by field survey, clearly indicated expected critical boundary of unsafe pollutant concentration from each of the outfall. It helped to identify the zone of influence of pollutant distribution from each outfall. Along the bank, the directional tidal flow was more prominent. Safe and unsafe zones on the riverbed were visibly demarcated by dark color and obnoxious odor. DO and BOD, the two important water quality parameters were taken into consideration for this interpolation. In between safe and unsafe boundary point the linear interpolation of DO and BOD value of river water helped to identify the zones where it reached its safest point seasonally. Water samplings were done by non-anchored country boat.
The locations were marked using Global Positioning System (GPS) and distances were calculated by location points. Environmental standards (Table B in
Attempt was made to measure the discharge by Hydrographic Survey of river Ganga at Ballykhal and Dakshineswar Canal by an instrument called ADCP (Acoustic Doppler current profiler) (Fig 1 in
The cross-sectional area of each stretch was calculated considering the width of the strip and the average depth of the same. This area of the strip was multiplied by the measured depth averaged velocity to obtain discharge value. Strip-wise depth average discharge at the sampling points was obtained by linear interpolation. The ultimate concentration of parameters of a mixture of stream water and wastewater were also estimated by using Plug flow model by applying
Where,
L0 = Ultimate BOD of The Mixture of Stream Water and Wastewater (mg/L)
Lr = Ultimate BOD of the River just Upstream of the Point of Discharge (mg/L)
Lw = Ultimate BOD of The Wastewater (mg/L)
Qr = Volumetric Flow Rate of The River Just Upstream of The Discharge Point (m3/s)
Qw = Volumetric Flow Rate of Wastewater (m3/s)
The exact coordinates of sampling points were obtained using GARMIN GPS. Seasonal changes of DO and BOD (during 2015–2018) at Dakshineswar Canal were considered to prepare water quality map by using Inverse Distance Weighted (IDW) interpolation technique in Arc GIS10 software considering its tidal variation. A map was developed on the basis of concentration of DO and BOD at different sampling points at Dakshineswar Canal.
Parameters like color, odor, pH, temperature, and DO were analyzed in the field immediately. The color of wastewater samples from all outfalls varied from light to dark brown. An unpleasant odor was a general characteristic of wastewater at all sampling points. At Khardah Khal, it was sensed from a few meters away of sampling locations which helped to track its location. The presence of organic debris, leaves, other wastes from industries, domestic sources made it unfit for use. The unpleasant odor may be due to the presence of metals, salts, alkaline materials, and end product of biological reactions [
Out of twenty outfalls, five were screened and considered for further experimental purposes. The value of DO was found lower than its standard limit in all outfalls except Rashbari and Bottala. The value of BOD crossed its standard limit at all selected outfalls. The concentration of nitrate nitrogen also crossed its standard limit at Annapurna Ghat, Circular Canal, Jaymataji Ghat, Cossipore area I, Dakshineswar Canal, New Jetty, Khardah Khal, Rashbari, and Ballykhal. The level of total and fecal coliform were several times higher than its standard value for each sampling locations. An alarming level of bacterial population was observed at Circular Canal, Dakshineswar Canal, New Jetty at Dakshineswar, Khardah Khal, and Ballykhal. The value of Pb and Hg was beyond its standard limit at Dakshineswar Canal and Khardah Khal. The alarming condition was also prevailing for Pb and Hg concentration at Ghusuri and Ballykhal. Five major outfalls were selected based on significant level of major water quality parameters, wastewater flow rate, and their location. A high wastewater flow rate i.e., a maximum of 356.40 million litres per day (MLD) during pre-monsoon and 138.24 MLD during post-monsoon were reported at Khardah Khal. Even the level of Hg highlighted the need for further analysis of wastewater sample from Khardah Khal. A high flow rate of wastewater was also reported at Circular Canal (320.3 MLD) and Ballykhal (581.33 MLD during pre-monsoon and 260.68 MLD for post-monsoon) (Tables C and D in
A seasonal analysis (pre-monsoon, monsoon, and post-monsoon) of water quality parameters of five major selected outfalls i.e., Circular Canal, Ghusuri, Dakshineswar Canal, Ballykhal, and Khardah Khal was performed, and mean value of each parameter was taken into consideration for statistical analysis (Table F in
Pre-monsoon value of DO, BOD, nitrate-nitrogen and chloride were significantly higher in comparison to monsoon, as evidenced by t-test analysis. The value of DO was significantly decreased during pre-monsoon in comparison to post-monsoon at Ghusuri, Dakshineswar Canal, and Khardah Khal. A significant increase of value of conductivity was reported at all sampling locations except Ghusuri during pre-monsoon in comparison to monsoon. The increasing trend of conductivity and BOD was also observed at Dakshineswar Canal and Khardah Khal when it was compared to post-monsoon. The level of nitrate nitrogen was increased significantly during pre-monsoon in comparison to post-monsoon for all sampling location except Ballykhal. A significantly higher value of total hardness was observed at Ghusuri, Ballykhal, and Khardah Khal during pre-monsoon than monsoon. At Ghusuri and Ballykhal, the level of total hardness and phosphorus was notably high during pre-monsoon in comparison to post-monsoon and monsoon respectively. Considering post-monsoon value, the level of total hardness was also significantly high during pre-monsoon at Ghusuri and Ballykhal. A significant higher value of chloride was also observed at Ghusuri and Khardah Khal during pre-monsoon than post-monsoon (
Outfalls | p-value | |||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
pH | Temperature (°C) | DO (mg/L) | Conductivity (μS/cm) | BOD (mg/L) | Total Hardness (mg/L) | Nitrate nitrogen (mg/L) | Chloride (mg/L) | Phosphorus (mg/L) | Total Coliform (MPN/100ml) | Fecal Coliform (MPN/100ml) | ||||||||||||
Pre-monsoon | Pre-monsoon | Pre-monsoon | Pre-monsoon | Pre-monsoon | Pre-monsoon | Pre-monsoon | Pre-monsoon | Pre-monsoon | Pre-monsoon | Pre-monsoon | Pre-monsoon | Pre-monsoon | Pre-monsoon | Pre-monsoon | Pre-monsoon | Pre-monsoon | Pre-monsoon | Pre-monsoon | Pre-monsoon | Pre-monsoon | Pre-monsoon | |
Vs | Vs | Vs | Vs | Vs | Vs | Vs | Vs | Vs | Vs | Vs | Vs | Vs | Vs | Vs | Vs | Vs | Vs | Vs | Vs | Vs | Vs | |
Monsoon | Post-monsoon | Monsoon | Post-monsoon | Monsoon | Post-monsoon | Monsoon | Post-monsoon | Monsoon | Post-monsoon | Monsoon | Post-monsoon | Monsoon | Post-monsoon | Monsoon | Post-monsoon | Monsoon | Post-monsoon | Monsoon | Post-monsoon | Monsoon | Post-monsoon | |
Circular Canal | 0.624 | 1 | 0.164 | 0.000* | 0.003* | 0.398 | 0.008* | 0.105 | 0.000* | 0.006 | 0.104 | 0.683 | 0.000* | 0.001* | 0.000* | 0.001 | 0.017 | 0.057 | 0.001* | 0.002* | 0.001* | 0.00* |
Ghusuri | 0.1 | 0.134 | 0.189 | 0.000* | 0.001 | 0.000* | 0.161 | 0.529 | 0.021* | 0.184 | 0.001* | 0.012* | 0.002* | 0.001* | 0.011* | 0.011* | 0.011* | 0.051 | 0.001* | 0.005* | 0.000* | 0.003* |
Dakshineswar Canal | 0.624 | 0.541 | 0.018 | 0.000* | 0.000* | 0.000* | 0.000* | 0.019* | 0.000* | 0.009* | 0.088 | 0.286 | 0.000* | 0.021* | 0.002* | 0.261 | 0.08 | 0.772 | 0.001* | 0.004* | 0.000* | 0.006* |
Ballykhal, | 0.001 | 0.006* | 0.737 | 0.000* | 0.002* | 0.856 | 0.004* | 0.104 | 0.028* | 0.382 | 0.000* | 0.001* | 0.006* | 0.494 | 0.000* | 0.054 | 0.000* | 0.106 | 0.001* | 0.002* | 0.001* | 0.001* |
Khardah Khal | 0.391 | 0.401 | 0.261 | 0.000* | 0.019* | 0.010* | 0.000* | 0.039* | 0.010* | 0.030* | 0.044* | 0.149 | 0.006* | 0.005* | 0.000* | 0.002* | 0.119 | 0.203 | 0.001* | 0.003* | 0.000* | 0.001* |
N = 16 * p value <0.05.
The level of total and fecal coliform was beyond its standard limit during seasonal analysis at all sampling sites. An over range of DO, BOD, nitrate nitrogen, and phosphorus was reported in most of the sampling sites for all three seasons. The presence of heavy metals was very much site specific, may be due to its surrounding sources. No spatial and seasonal impact on pH was observed and the temperature changed seasonally. In the study area, wastewater from outfalls mainly settle within certain distance which was directly proportional to the tidal actions. So, during monsoon and post-monsoon, when the excess rainwater mixed with mainstream, the density of pollution in water becomes diluted. However, during pre-monsoon due to minimization of volume of water, the level of pollution may be high. These may be the reason for water quality variation in different seasons. The same kind of trend was observed in many works on river Ganga [
The correlation matrix was generated separately for each sampling site considering all measured parameters except color and odor (Tables G-K in
Name of the site | y (Dependent) | x (Independent) | Regression Equation | The value of R2 | Calculated | Actual value (pre-monsoon, 2019) | Percentage of Error |
---|---|---|---|---|---|---|---|
Circular Canal | Nitrate nitrogen (NN) | Chloride (CL) | NN = 0.4023CL + 1.0627 | 0.9161 | 14.339 | 14.5 | -1.11 |
Nitrate nitrogen (NN) | Lead (Pb) | NN = 91.95Pb + 6.2611 | 0.9194 | 14.537 | 14.5 | 0.25 | |
Nitrate nitrogen (NN) | Total Arsenic (As) | NN = 536.54As + 4.9135 | 0.9737 | 13.498 | 14.5 | -6.91 | |
Chloride (CL) | Lead (Pb) | CL = 220.05Pb + 13.279 | 0.93 | 33.084 | 33.0 | 0.25 | |
Chloride (CL) | Mercury (Hg) | CL = 3586.5Hg + 6.0284 | 0.9705 | 34.720 | 33.0 | 5.21 | |
Lead (Pb) | Mercury (Hg) | Pb = 15.422Hg—0.0289 | 0.9343 | 0.094 | 0.090 | 4.97 | |
Lead (Pb) | Total Coliform (TC) | Pb = 1E-08TC—0.0136 | 0.9762 | 0.056 | 0.090 | -37.33 | |
Lead (Pb) | Fecal Coliform (FC) | Pb = 6E-08FC—0.0138 | 0.9682 | 0.070 | 0.090 | -22.00 | |
Total Coliform (TC) | Fecal Coliform (FC) | TC = 4.7974FC—15448 | 0.9886 | 6700912 | 7000000 | -4.27 | |
Ghusuri | Total Hardness (HAR) | Total Arsenic (As) | HAR = 5981.9As + 115.83 | 0.919 | 187.6 | 184 | 1.96 |
Total Hardness (HAR) | Fecal Coliform (FC) | HAR = 0.0006FC+ 95.442 | 0.9006 | 179.4 | 184 | -2.48 | |
Total Arsenic (As) | Total Coliform (TC) | As = 2E-08TC—0.0033 | 0.9541 | 0.0107 | 0.012 | -10.83 | |
Total Arsenic (As) | Fecal Coliform (FC) | As = 1E-07FC—0.0033 | 0.9575 | 0.0107 | 0.012 | -10.83 | |
Total Coliform (TC) | Fecal Coliform (FC) | TC = 5.0112FC + 2622.6 | 0.9866 | 704190.6 | 700000 | 0.60 | |
Dakshineswar | Total Coliform (TC) | Fecal Coliform (FC) | TC = 4.9371FC + 15809 | 0.9575 | 8798024 | 7500000 | 17.31 |
Lead (Pb) | Mercury (Hg) | Pb = 10.692 Hg—0.0125 | 0.9161 | 0.06 | 0.05 | 24.69 | |
BOD | Total Arsenic (As) | BOD = 299.29As + 3.2648 | 0.9385 | 6.856 | 7.9 | -13.21 | |
BOD | Total Coliform (TC) | BOD = 6E-07TC + 2.7545 | 0.9245 | 7.255 | 7.9 | -8.17 | |
BOD | Fecal Coliform (FC) | BOD = 3E-06FC + 2.8208 | 0.9051 | 8.071 | 7.9 | 2.16 | |
Ballykhal | Conductivity (CON) | Total Arsenic (As) | CON = 37152As+ 373.07 | 0.9023 | 818.9 | 817.3 | 0.20 |
Total Hardness (HAR) | Total Arsenic (As) | HAR = 11449As + 101.07 | 0.9468 | 238.5 | 244.0 | -2.27 | |
Lead (Pb) | Mercury (Hg) | Pb = 8.0764Hg—0.0119 | 0.9504 | 0.045 | 0.048 | -7.01 | |
Lead (Pb) | Total Arsenic (As) | Pb = 4.189As—0.0052 | 0.9298 | 0.045 | 0.048 | -6.11 | |
Mercury (Hg) | Total Arsenic (As) | Hg = 0.5006As + 0.001 | 0.9112 | 0.007 | 0.007 | 0.10 | |
Mercury (Hg) | Total Coliform (TC) | Hg = 9E-10TC + 0.0013 | 0.9425 | 0.008 | 0.007 | 15 | |
Mercury (Hg) | Fecal Coliform (FC) | Hg = 4E-09FC + 0.0014 | 0.9109 | 0.0078 | 0.007 | 11.43 | |
Total Coliform (TC) | Fecal Coliform (FC) | TC = 4.9373FC—8016 | 0.9854 | 7891664 | 7500000 | 5.22 | |
Khardah Khal | Mercury (Hg) | Total Arsenic (As) | Hg = 0.4989As + 0.0033 | 0.9511 | 0.012 | 0.012 | -1.82 |
Mercury (Hg) | Total Coliform (TC) | Hg = 1E-09TC + 0.0021 | 0.9143 | 0.009 | 0.012 | -24.17 | |
Mercury (Hg) | Fecal Coliform (FC) | Hg = 6E-09FC + 0.0019 | 0.9517 | 0.010 | 0.012 | -14.17 | |
Total Arsenic (As) | Total Coliform (TC) | As = 2E-09TC- 0.002 | 0.903 | 0.012 | 0.017 | -29.41 | |
Total Coliform (TC) | Fecal Coliform (FC) | TC = 5.1002FC + 55729 | 0.9796 | 7196009 | 7000000 | 2.80 |
When the value of R2 remain 0.999, the result can be elaborated in a way that there is a chance of 99.9% variability of water quality related to respective parameters [
The analysis of variation of water quality parameters due to tidal fluctuation was done season wise for five selected outfalls as well. It helped to spot the distance of safe zone from the outfalls where parameters reach their standard value. The prediction of concentration of parameters was also done at several points from Dakshineswar Canal and Ballykhal.
The distance wise changes of concentration of parameters were obtained from direction wise analysis for five separate outfalls during tidal fluctuation (Tables L-R in
Seasons | Locations | BOD (mg/L) | DO (mg/L) | Nitrate nitrogen (mg/L) | Phosphorous (mg/L) | ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Across the River (m) | Diagonally from The Outfall (m) | Along the Bank (m) | Across the River (m) | Diagonally from The Outfall (m) | Along the Bank (m) | Across the River (m) | Diagonally from The Outfall (m) | Along the Bank (m) | Across the River (m) | Diagonally from The Outfall (m) | Along the Bank (m) | ||
Pre-monsoon | CIRCULAR CANAL | 71.1 | 81 | 128.7 | 98.4 | 107.4 | 108.6 | 89.4 | 81.9 | 105.9 | 74.3 | 82.3 | 68.6 |
GHUSURI | 36.7 | 34.1 | 45.8 | 44.7 | 48.4 | 46.1 | 31.8 | 34.9 | 44.8 | 24.5 | 24 | 21.7 | |
DAKSHINESWAR CANAL | 75.6 | 78.5 | 90.9 | 87.1 | 85.3 | 81.6 | 59.9 | 76.1 | 86.3 | 72.1 | 73.3 | 70.5 | |
BALLYKHAL | 69.1 | 91.3 | 138.4 | 125 | 111.8 | 142.1 | 52.3 | 77.3 | 120.6 | 74.2 | 83.7 | 105.1 | |
KHARDAH KHAL | 77.2 | 78.7 | 127.1 | 85.2 | 76.8 | 115.4 | 44.2 | 43.8 | 67.4 | 37.1 | 38.4 | 55.1 | |
Monsoon | CIRCULAR CANAL | 50 | 67.8 | 80.5 | 2.3 | 7.5 | 7.4 | -- | -- | -- | -- | -- | -- |
GHUSURI | 14.7 | 20.2 | 31.2 | -- | -- | -- | -- | -- | -- | -- | -- | -- | |
DAKSHINESWAR CANAL | 18 | 15.2 | 16.2 | 17.4 | 20.3 | 21.8 | 3 | 2.7 | 3.5 | 29.6 | 21.2 | 29.5 | |
BALLYKHAL | 23.8 | 27.7 | 33.1 | 6 | 6 | 7 | 19.5 | 27.6 | 30 | 15.2 | 16.2 | 20.4 | |
KHARDAH KHAL | 48.3 | 36.1 | 49.3 | 27.5 | 28.8 | 39.1 | 13 | 10.6 | 14.6 | 22.9 | 22.4 | 33.2 | |
Post-monsoon | CIRCULAR CANAL | 52.8 | 72.1 | 88.8 | 79.9 | 116.6 | 134.5 | -- | -- | -- | 27.8 | 33.4 | 49.6 |
GHUSURI | 27.3 | 32.6 | 38.1 | 31.3 | 36.7 | 36.6 | -- | -- | -- | 12 | 22.2 | 32.3 | |
DAKSHINESWAR CANAL | 29.5 | 30 | 46.1 | 69.1 | 62.4 | 80.7 | 37.8 | 36 | 35 | 37.5 | 33 | 37.9 | |
BALLYKHAL | 55.2 | 56.6 | 76.4 | 121.6 | 116.9 | 140.6 | 16.7 | 41.9 | 48.3 | 35.2 | 39.6 | 49.5 | |
KHARDAH KHAL | 40.2 | 39.9 | 56.6 | 84 | 73.4 | 122.9 | 23.8 | 23.8 | 31.9 | 27 | 28 | 38.4 |
The influence of each outfall on river water quality varied with its flow and tidal dynamics. During low tide, the concentration of parameters was quite high in comparison to high tide. DO, BOD, nitrate nitrogen, and phosphorous are the most influential parameters which changed with distances and directions. The rate of changes in their concentrations was recorded maximum with increasing distance from outfalls during low tide. High tide data did not demonstrate any notable impact on pollutants distribution as the level of parameters remained within its standard value at its entry point in river Ganga only.
During pre-monsoon, the influence was maximum in comparison to other two seasons for all sampling locations except at Circular Canal and Khardah Khal where the impact of DO remain comparatively high during post-monsoon. The concentration of parameter reached its limit by travelling a shorter distance during monsoon may be due to high water level in river Ganga. The study demonstrated a prominent influence of DO, BOD, nitrate nitrogen, and phosphorus at Ballykhal followed by Khardah Khal and Circular Canal. The minimum influence was reported at Ghusuri. Other than DO, BOD, nitrate nitrogen, and phosphorus, all other measured parameters reached their standard value at source of wastewater discharging point only, so they were not considered for calculation. The increasing level of water in river Ganga enhances the dilution effect which changes the level of parameters. The reason behind minimizing BOD level or increasing DO value during monsoon may be the dilution effect. The trend of sedimentation of the phosphorus was directly proportional to the total volume of the water in riverbed and may be responsible for minimizing its value in monsoon as well. At Ballykhal, Khardah Khal, and Circular Canal, the discharge of a large volume of wastewater may be responsible for its maximum influence. The volume of wastewater reflected its zone of influence. Higher the concentration, the longer the path it needed to travel to meet its criteria for standard. At Ghusuri, a comparatively low volume of wastewater release reduced its path. Not only the wastewater flow, discharge rate of river which varied with tidal fluctuation was highly responsible for its distribution pattern. No variation of concentration of parameters with distance was observed during high tide in any of sampling spots. During high tide the value remained more or less constant and for most of the cases it met its standard limit at outfalls only (Figs 4–30 in
A model-based validation (the point source, plug flow model for dissolved oxygen calculation) of pre-monsoon water quality parameters of Dakshineswar Canal and Ballykhal was done by using strip-wise depth average discharge (
Distance (m) | Ballykhal | |||
---|---|---|---|---|
Discharge (m3/s) | Discharge (m3/s) | Discharge (m3/s) | Discharge (m3/s) | |
(Across the river directly from outfall during low tide) | (Across the river from outfall during high tide) | (Across the river in upstream of outfall during low tide) | (Across the river in upstream of outfall during high tide) | |
0 | 0.017 | 0.023 | 0.071 | 0.0862 |
15 | 0.078 | 0.22 | 0.122 | 0.1582 |
35 | 0.186 | 0.232 | 0.19 | 0.2542 |
55 | 0.269 | 0.29 | 0.258 | 0.3502 |
75 | 0.346 | 0.484 | 0.326 | 0.4462 |
95 | 0.402 | 0.563 | 0.394 | 0.5422 |
115 | 0.492 | 0.689 | 0.462 | 0.6382 |
135 | 0.546 | 0.764 | 0.53 | 0.7342 |
155 | 0.625 | 0.875 | 0.598 | 0.8302 |
175 | 0.705 | 0.987 | 0.666 | 0.9262 |
Dakshineswar Canal | ||||
0 | 0.15 | 0.191 | 0.28 | 0.252 |
20 | 0.19 | 0.26 | 0.298 | 0.306 |
40 | 0.22 | 0.342 | 0.316 | 0.36 |
60 | 0.265 | 0.393 | 0.334 | 0.414 |
80 | 0.276 | 0.423 | 0.352 | 0.468 |
100 | 0.313 | 0.458 | 0.37 | 0.522 |
120 | 0.362 | 0.492 | 0.388 | 0.576 |
140 | 0.376 | 0.532 | 0.406 | 0.63 |
180 | 0.432 | 0.602 | 0.442 | 0.738 |
200 | 0.49 | 0.191 | 0.46 | 0.792 |
Distance from outfall (m) across the river | Low Tide | Distance from outfall (m) across the river | High Tide | ||
Discharge (m3/s) (Across the river) | Discharge (m3/s) (Across the river just upstream) | Discharge (m3/s) (Across the river) | Discharge (m3/s) (Across the river just upstream) | ||
45.7 | 0.215 | 0.2 | 11.39 | 0.122 | 0.141 |
70.3 | 0.311 | 0.349 | 36.09 | 0.253 | 0.259 |
128.8 | 0.54 | 0.499 | 70.17 | 0.434 | 0.423 |
Dakshineswar Canal | |||||
Distance from outfall (m) across the river | Low Tide | Distance from outfall (m) across the river | High Tide | ||
Discharge (m3/s) (Across the river) | Discharge (m3/s) (Across the river just upstream) | Discharge (m3/s) (Across the river) | Discharge (m3/s) (Across the river just upstream) | ||
42.22 | 0.223 | 0.354 | 32.22 | 0.305 | 0.372 |
66.41 | 0.262 | 0.276 | 53.98 | 0.346 | 0.342 |
100.21 | 0.316 | 0.395 | 89.56 | 0.414 | 0.512 |
Parameters | Ballykhal | Dakshineswar Canal | ||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Lowtide | High Tide | Lowtide | High Tide | |||||||||||||
Distance (m) | Estimated Value | Measured Value | Error % | Distance (m) | Estimated Value | Measured Value | Error % | Distance (m) | Estimated Value | Measured Value | Error % | Distance (m) | Estimated Value | Measured Value | Error % | |
BOD (mg/L) | 45.7 | 4.7 | 4.1 | 12.56 | 11.4 | 1.3 | 1.4 | -3.98 | 42.22 | 4.3 | 4.7 | -9.81 | 32.22 | 2.8 | 2.9 | -4.43 |
70.3 | 3.4 | 3 | 11.08 | 36.1 | 1.3 | 1.3 | 1.73 | 66.41 | 3.1 | 3.9 | -25.42 | 53.98 | 2.6 | 2.5 | 3.03 | |
128 | 2.8 | 2.1 | 25.26 | 70.2 | 1.3 | 1.3 | 0.89 | 100.21 | 2.5 | 2.2 | 11.75 | 89.56 | 2.4 | 2.2 | 8.11 | |
DO (mg/L) | 45.7 | 3.6 | 2.6 | 28.58 | 11.4 | 7.2 | 7.3 | -1.95 | 42.22 | 4.9 | 2.7 | 44.64 | 32.22 | 6.1 | 6.2 | -2.43 |
70.3 | 4.5 | 4.1 | 9.24 | 36.1 | 7.2 | 7.2 | 0.43 | 66.41 | 5.7 | 4.2 | 26.45 | 53.98 | 6.1 | 6.2 | -1.01 | |
128 | 4.9 | 5.1 | -4.22 | 70.2 | 7.3 | 7.3 | -0.48 | 100.21 | 6.1 | 6.5 | -5.71 | 89.56 | 6.2 | 6.2 | 0.27 | |
CHORIDE (mg/L) | 45.7 | 28 | 37 | -32.08 | 11.4 | 14.7 | 15.2 | -3.75 | 42.22 | 37.4 | 46 | -22.93 | 32.22 | 15.1 | 15.3 | -1.16 |
70.3 | 21.2 | 15.6 | 26.37 | 36.1 | 14.3 | 14.6 | -1.95 | 66.41 | 26 | 37.7 | -44.92 | 53.98 | 14.8 | 15.3 | -3.1 | |
128 | 18.3 | 15.2 | 16.77 | 70.2 | 14.2 | 14.5 | -2.38 | 100.21 | 20 | 19.2 | 4.02 | 89.56 | 14.6 | 14.3 | 1.9 | |
CONDUCTIVITY (μS/cm) | 45.7 | 674.17 | 736.7 | -9.27 | 11.4 | 489.8 | 448.7 | 8.39 | 42.22 | 779.85 | 780.3 | -0.06 | 32.22 | 311.36 | 205 | 34.16 |
70.3 | 592.69 | 631 | -6.46 | 36.1 | 505.1 | 443.7 | 12.15 | 66.41 | 585.46 | 357.3 | 38.97 | 53.98 | 273.16 | 205 | 24.95 | |
128 | 557.76 | 589.7 | -5.73 | 70.2 | 512.5 | 443.3 | 13.49 | 100.21 | 430.68 | 289 | 32.9 | 89.56 | 237.81 | 205 | 13.8 | |
TOTAL HARDNESS (mg/L) | 45.7 | 199.7 | 225 | -12.66 | 11.4 | 167.5 | 185.7 | -10.89 | 42.22 | 200.9 | 268.7 | -33.75 | 32.22 | 202.5 | 196.7 | 2.85 |
70.3 | 174.5 | 212 | -21.49 | 36.1 | 159.6 | 184 | -15.26 | 66.41 | 174.3 | 223 | -27.97 | 53.98 | 198.9 | 195.7 | 1.59 | |
128 | 163.7 | 172 | -5.08 | 70.2 | 155.9 | 188 | -20.62 | 100.21 | 160.2 | 152 | 5.13 | 89.56 | 195.5 | 195.7 | -0.09 | |
NITRATE NITROGEN (mg/L) | 45.7m | 11.5 | 10.2 | 10.95 | 11.4 | 4.9 | 3.5 | 29.23 | 42.22 | 13 | 12.5 | 4.11 | 32.22 | 6.6 | 6.1 | 7.5 |
70.3m | 8.7 | 8.9 | -2.56 | 36.1 | 5.6 | 3.5 | 37.28 | 66.41 | 10 | 9.2 | 8.36 | 53.98 | 6.4 | 6.1 | 5.05 | |
128m | 7.5 | 6.5 | 13.19 | 70.2 | 5.9 | 3.5 | 40.54 | 100.21 | 8.5 | 9 | -6.36 | 89.56 | 6.3 | 6.1 | 2.66 | |
PHOSPHORUS (mg/L) | 45.7m | 8.8 | 6.1 | 30.39 | 11.4 | 4.6 | 5.1 | -10.44 | 42.22 | 8 | 7.2 | 9.78 | 32.22 | 1.9 | 1.7 | 8.84 |
70.3m | 6.4 | 5.8 | 8.69 | 36.1 | 4.4 | 5.1 | -15.74 | 66.41 | 5.9 | 5.8 | 2.31 | 53.98 | 1.8 | 1.7 | 5.98 | |
128m | 5.3 | 4.3 | 19.14 | 70.2 | 4.3 | 5.1 | -18.48 | 100.21 | 4.9 | 4.2 | 13.59 | 89.56 | 1.8 | 1.7 | 3.16 |
GIS based mapping is a widely accepted tool for assessing water quality parameters [
Spatial distribution map of BOD at Dakshineswar Canal during pre-monsoon low tide (left) and pre-monsoon high tide (right).
Spatial distribution map of BOD at Dakshineswar Canal during monsoon low tide (left) and monsoon high tide (right).
Spatial distribution map of BOD at Dakshineswar Canal during post-monsoon low tide (left) and post-monsoon high tide (right).
Spatial distribution map of DO at Dakshineswar Canal during pre-monsoon low tide (left) and pre-monsoon high tide (right).
Spatial distribution map of DO at Dakshineswar Canal during monsoon low tide (left) and monsoon high tide (right).
Spatial distribution map of DO at Dakshineswar Canal during post-monsoon low tide (left) and post-monsoon high tide (right).
A comparison between low tide and high tide map clearly showed that high level of BOD and low level of DO prevail during low tide than high tide. A prominent change of level of parameters was reported with increasing distances from the outfall at all directions. During monsoon, the changes were more stable but more prominent change was reflected in pre-monsoon map (Figs
The extensive field survey throughout the study area helped to screen five major outfalls out of all twenty: Circular Canal, Ghusuri, Dakshineswar canal, Ballykhal, and Khardah Khal based on its concentration of parameters. Seasonal analysis of four consecutive years showed a significant difference in concentration of major water quality parameters in pre-monsoon in comparison to post-monsoon and monsoon periods in all sampling locations. Seasonal variation was also prominently reflected by GIS mapping based on water quality parameters from different part of the river considering the position of outfalls. Distance and direction wise analysis of wastewater from outfalls indicated the prominent influence of wastewater along the bank of the river from each outfall. A strong correlation in between heavy metals and total coliform, and fecal coliforms were established in most of the sampling sites. Regression analysis helped to predict the level of one parameter by measuring other with which it is highly correlated to.
Considering the location of the outfalls, not only the residential sources, the influence of the release from small and medium scale industries cannot be ignored in most of the cases. The study encouraged the source reduction i.e., to restrict the release of untreated wastewater into the river. The abstraction of river water from particular distance from outfalls can reduce the chances of contamination. It is not like that river water is unfit to use everywhere but on the basis of intensity of outfalls, its direction of flow, discharge rate and the tidal movement, the influence can be easily predicted and a guideline to use river water can be generated in particular outfalls areas.
S1 Table A. Methods of measuring water quality parameters [
(DOCX)
S2 Fig1. ADCP (Acoustic Doppler current profiler) device. S2 Fig2. Identification of Outfalls in Left Bank of the river Ganga from South to North during 2014 from Howrah Station to Khardah, 24 Parganas (North), West Bengal, India. S2 Fig3. Identification of Outfalls in Right Bank of the river Ganga from South to North during 2014 from Howrah Station to Khardah, 24 Parganas (North), West Bengal, India. S2 Fig4. Zone of influence of DO, BOD, Nitrate nitrogen and Phosphorus at Circular Canal during pre-monsoon low tide. S2 Fig5. Zone of influence of DO and BOD at Circular Canal during monsoon low tide. S2 Fig6. Zone of influence of DO, BOD and Phosphorus at Circular Canal during post-monsoon low tide. S2 Fig7. Zone of influence of DO, BOD, Nitrate nitrogen and Phosphorus at Ghusuri during pre-monsoon low tide. S2 Fig8. Zone of influence of BOD at Ghusuri during monsoon low tide. S2 Fig9. Zone of influence of DO, BOD and Phosphorus at Ghusuri during post-monsoon low tide. S2 Fig10. Zone of influence of DO, BOD, Nitrate nitrogen and Phosphorus at Dakshineswar Canal during pre-monsoon low tide. S2 Fig11. Zone of influence of DO, BOD, Nitrate nitrogen and Phosphorus at Dakshineswar Canal during monsoon low tide. S2 Fig12. Zone of influence of DO, BOD, Nitrate nitrogen and Phosphorus at Dakshineswar Canal during post-monsoon low tide. S2 Fig13. Zone of influence of DO, BOD, Nitrate nitrogen and Phosphorus at Ballykhal during pre-monsoon low tide. S2 Fig14. Zone of influence of DO, BOD, Nitrate nitrogen and Phosphorus at Ballykhal during monsoon low tide. S2 Fig15. Zone of influence of DO, BOD, Nitrate nitrogen and Phosphorus at Ballykhal during post-monsoon low tide. S2 Fig16. Zone of influence of DO, BOD, Nitrate nitrogen and Phosphorus at Khardah Khal during pre-monsoon low tide. S2 Fig17. Zone of influence of DO, BOD, Nitrate nitrogen and Phosphorus at Khardah Khal during monsoon low tide. S2 Fig18. Zone of influence of DO, BOD, Nitrate nitrogen and Phosphorus at Khardah Khal during post-monsoon low tide. S2 Fig19. Zone of influence of BOD at selected outfalls during pre-monsoon low tide. S2 Fig20. Zone of influence of BOD at selected outfalls during monsoon low tide. S2 Fig21. Zone of influence of BOD at selected outfalls during post-monsoon low tide. S2 Fig22. Zone of influence of DO at selected outfalls during pre-monsoon low tide. S2 Fig23. Zone of influence of DO at selected outfalls during monsoon low tide. S2 Fig24. Zone of influence of DO at selected outfalls during post-monsoon low tide. S2 Fig25. Zone of influence of Nitrate nitrogen at selected outfalls during pre-monsoon low tide. S2 Fig26. Zone of influence of Nitrate nitrogen at selected outfalls during monsoon low tide. S2 Fig27. Zone of influence of Nitrate nitrogen at selected outfalls during post-monsoon low tide. S2 Fig28. Zone of influence of Phosphorus at five selected outfalls during pre-monsoon low tide. S2 Fig29. Zone of influence of Phosphorus at five selected outfalls during monsoon low tide. S2 Fig30. Zone of influence of Phosphorus at five selected outfalls during post-monsoon low tide.
(DOCX)
The authors want to acknowledge the support provided by Asutosh College, Kolkata, and Jadavpur University, Kolkata for conducting this study. Also, the authors want to acknowledge the support provided by Navajo Technical University towards this study.