Small But Important: The Role of Small Floodplain Tributaries to River

Dec 5, 2017 - We present results from time series sampling campaigns over 2015 and 2016. ... runoff from small floodplain rivers is an important flux ...
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Article Cite This: ACS Earth Space Chem. XXXX, XXX, XXX−XXX

Small But Important: The Role of Small Floodplain Tributaries to River Nutrient Budgets Indra S. Sen,* Soumita Boral, Sudhakar Ranjan, and Sampat K. Tandon Department of Earth Sciences, Indian Institute of Technology Kanpur, Kanpur, UP 208016, India S Supporting Information *

ABSTRACT: The fertile Indo-Gangetic floodplain contains numerous small, rain-fed rivers. These rivers contribute to the river water chemistry of the Ganges River; however, these small floodplain rivers are never studied nor monitored owing to their smaller size with reference to catchment area (∼1000− 10000 km2) and volume of discharge (∼10−100 m3/s). Here we quantify the role of a small flood plain river, the Pandu River, in terms of dissolved inorganic nitrogen (DIN) and phosphate export to Ganges River. We present results from time series sampling campaigns over 2015 and 2016. Our result shows that Pandu River exports 793 ± 128 t/yr of DIN and 177 ± 29 t/yr phosphate to the Ganges River, which accounts for 0.1% and 0.42% of the total DIN and phosphate fluxes, respectively, that Ganges River exports into Bay of Bengal. Furthermore, we show that the small floodplain rivers in the Indo-Gangetic floodplain could collectively contribute ∼15% and ∼61% of the DIN and phosphate fluxes, respectively, that Ganges River delivers into Bay of Bengal. Therefore, runoff from small floodplain rivers is an important flux that could contribute to the dissolved nutrient budget of large river systems, and they must be better monitored to address future challenges in river basin management. KEYWORDS: nutrient fluxes, nitrate and phosphate contamination, N and P transport pathways in Indo-Gangetic floodplain, riverine nutrient budget, small floodplain rivers that ∼85% of the total wastewater generation (∼95 m3/s) from 222 towns in the entire Ganges basin is directly discharged without treatment into the Ganges River and its tributaries and ∼15% is disposed onto the land.9 Although direct industrial and municipal sewage discharges into the river can be constrained by implementing zero liquid discharge to the river, the latter will be harder to constrain. The other concern is the increasing rate of N- and P-based fertilizer application in the IndoGangetic floodplains, and most of the fertilizer consumption happens during winter period when the rivers are at a low flow stage (Figure 2). However, we have very limited understanding on what happens to the N and P coming out of fertilizers, their internal cycling, and transport pathways in the Indo-Gangetic floodplains. Fertilizer derived N and P can enter the aquatic environment via groundwater discharge (baseflow) and surface runoff10 and can act as a limiting nutrient.11 Given that a direct relationship between fertilizer usage, population growth, and riverine N and P fluxes12−14 has been observed, nutrient export through nonpoint pollution sources such as small floodplain tributaries will be an emerging concern in the Indo-Gangetic floodplains.

1. INTRODUCTION Rivers form the backbone of our society as they provide water and food security. However, anthropogenic activities are adversely affecting the health of rivers globally to an extent that many rivers are at a tipping point,1 and further changes will likely be rapid and unpredictable. River restoration projects are therefore ongoing in many river basins around the world to address future challenges in river basin management.2−4 On the Indian subcontinent, the Ganges River is the focal point of all cleaning and river restoration projects as the river provides water and food security to half a billion people.5 Many national and international initiatives have been undertaken to restore the Ganges River health, such as “Zero liquid discharge” initiative. Although zero liquid discharge to the river exclusively focused on point sources to address many river health issues, contributions from nonpoint sources still persist. Contributions from nonpoint sources are generally important for elements such as N and P when compared to point sources.6 An important nonpoint pollution source to the Ganges River is small floodplain rivers (Figure 1). However, these rivers are rarely studied since they are small in catchment area and discharge. As a result, the impact of these small floodplain rivers on the dissolved chemical load of large river systems is not constrained. The dominant source of anthropogenic N and P in rivers is thought to be contributions from large and small-scale industries, discharge of domestic and industrial sewage, and extensive use of fertilizers on agricultural land.7,8 It is estimated © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

September 29, 2017 December 3, 2017 December 4, 2017 December 5, 2017 DOI: 10.1021/acsearthspacechem.7b00112 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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ACS Earth and Space Chemistry

Figure 1. Examples of few small flood plain rivers that are tributaries of Ganges River in the Indian flood plains. The figure shows the following rivers and their catchment sizes in km2 are given in parentheses: Choya (382), Fulhara (13537), Garra (7334), GT1 (239), GT2 (143), Kali (328), Kalkaliya (690), Karamnasa (8314), kiul (27668), Mahananda (123322), Malini (883), Pandu (1495), Preshak (459), Tamsa (10171), Thora (1534), Varuna (3705), and Tauns (18244). The catchment areas inside parentheses were calculated using Arc GIS software. Total catchment area of these small floodplain rivers is 218448 km2. The small inset in panel 5 compares the length, drainage area, and discharge of Pandu River with some of the major rivers of the world.63 The map was created using ArcGIS 10.0

Figure 2. Annual urea application in Ganges Basin between 2007 and 2016 (black axis) and monthly variation of urea consumption for 2015 in Pandu Basin (orange axis).61

land-to-ocean riverine fluxes can be found at the United Nations Environment Program, Global Environmental Monitoring System, Global River Inputs (GEMS/GLORI) database25 and from the International program on Land−Ocean Interactions in the Coastal Zone (LOICZ).26,27 Many of these rivers are well studied, and some have even been continuously monitored over the last few decades.28 More recently, some rivers have been monitored for nutrients with sensor networks,

1.1. N and P Transport from Small Floodplain Rivers: The Missing Link. Concentrations, fluxes, and cycling of N and P are well studied in the ocean15−18 because P and N deficiency can limit primary production, whereas their overabundance can cause eutrophication, acidification, harmful algal blooms, and hypoxia.19−21 The ocean receives the majority of its P and N through river discharge, and as a result P and N transport by large rivers to the ocean is also very well studied.22−24 For example, a comprehensive data set of N−P B

DOI: 10.1021/acsearthspacechem.7b00112 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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respectively, of rain events. As our sampling was carried out only during nonrainy days, and baseflow and runoff from agricultural fields should be similar throughout the month, the use of a single data point representing a month is fair. Only for the month of July, the use of single data point may not be appropriate for assessing the representativeness of the month. It is likely that discharge generated from rain events may have diluted the concentrations of nutrients in the river, and therefore a more frequent sampling would have better quantified nutrient export during July. However, nutrient export during July could not be better constrained due to lack of available data. Therefore, because of our sampling strategy we envisage that some uncertainties may exist in the annual nutrient flux estimates. All samples were filtered using the technique outlined by Voss et al.44 The samples were frozen within 8 h of sample collection and were stored at −20 °C until analysis. Nutrient concentrations were analyzed using a Seal Analytical continuous-flow Auto Analyzer 3. Reagents and standards were freshly prepared prior to each analytical session. Ammonium was determined according to the Berthelot reaction technique45,46 and phosphate (soluble reactive phosphate) was measured according to the spectrophotometric molybdenumblue method.47−49 Soluble silicates in water samples were determined spectrophotometrically based on the reduction of silico-molybdate in acidic solution to molybdenum blue by ascorbic acid.50−52 Nitrite was determined by spectrophotometric detection of the diazo compounds.50,53 Concentration of ammonium, silicate, nitrite, and phosphate were determined against a ammonium, silicate, nitrite, and phosphate solution diluted to appropriate concentrations to generate a four-point calibration curve. All samples were run in duplicate and some in triplicate. The analytical precision for ammonium, silicate, nitrite, and phosphate were ±0.06, ±0.01, ±0.02, and ±0.012 μmol/L, respectively. Gravimetrically determined in-house standards were analyzed to assess the data quality. The measured ammonium, nitrite, silicate, and phosphate agree well with our in-house standards (Table S-1). Nitrate concentrations were measured using a portable YSI water quality probe. For the calibration of nitrate, we used standards provided by YSI with code YSI3885, YSI3886, and YSI 3887 having concentrations of 1, 10, and 100 mg/L with accuracy of ±1%. The probe was always calibrated for nitrate just prior to field visits. We report the summation of nitrate, nitrite, and ammonium concentrations as dissolved inorganic nitrogen (DIN). River discharge was measured using a SonTek Flow tracker instrument. The technique involves taking a series of velocity and depth measurements at several points across the stream channel. These measurements are combined with location and water depth information to compute the total discharge with Mid-Section discharge equation. The uncertainties associated with river discharge values were 7.5% as per the Flow Tracker Hand-held ADV Technical Manual. Nutrient fluxes were estimated using USGS LoadEstimator program (LOADEST) in the LoadRunner software package.54,55 LOADEST uses time series concentration and discharge data to construct a regression model, which is chosen from a suite of predetermined multiple regression models using Akaike’s information criterion. Here we have used adjusted maximum likelihood estimation (AMLE) values since the model residuals were normally distributed. For error, we have considered the standard error of prediction. For calculating the nutrient yield,

which report real-time data online (Land/Ocean Biogeochemical Observatory).29 Long-term records of N and P concentrations in river water have shown that human activities have significantly changed riverine loads of N and P.30,31 For example, Green et al.32 show that riverine export of DIN to the global ocean has increased 6fold from the preindustrial (N ∼ 2.1 × 106 t/yr) to the contemporary period (N ∼ 14.5 × 106 t/yr). Mitigation efforts have achieved some reduction of nutrient loads in certain areas. For example, better agricultural practices and improvements to wastewater treatment processes have reduced the average nitrate concentration of European rivers by 0.8% per year over the period 1992 to 201233 From a global perspective, our knowledge of riverine N and P concentrations, their temporal variability, and land-to-ocean fluxes in European rivers,34 Arctic rivers,35 Chinese rivers,36,37 and North American Rivers38 is comprehensive. On the contrary, the rivers draining the Indian subcontinent are less well studied, and most of the studies have focused on the large rivers.39−42 Recently Krishna et al.43 comprehensively reported the DIN and phosphate fluxes of 27 Indian peninsular rivers (catchment area ranged from 1000 to 31,300 km2) to the northern Indian Ocean. Their study shows that the monsoonal and glacial rivers in India export (1.84 ± 0.46) × 106, (0.28 ± 0.07) × 106 and (3.58 ± 0.89) × 106 t/yr of nitrate, phosphate, and silicate, respectively, to the Bay of Bengal. These fluxes are substantial when compared to other rivers, and the authors attributed the majority of N and P to anthropogenic sources. To the best of our knowledge, there are no studies that quantify fluxes of DIN and phosphate from small floodplain monsoonal rivers to large river systems such as the Ganges. To fill this gap we have monitored the Pandu River for one year between February 2015 and April 2016. Pandu River is 242 km long and is a right bank tributary of Ganges (Figure 1). It has a total catchment area of 1495 km2 and the catchment area is dominated by agricultural land (95%). Water samples were collected every month for dissolved ammonium, silicate, nitrite, nitrate, and phosphate concentrations. The main objective of the study was to quantify the export fluxes of DIN (nitrate + nitrite + ammonium) and phosphate from small Indo-Gangetic flood plain rivers, and understand its importance in large river nutrient budget.

2. METHODOLOGY All water samples were collected from the center of the rivers by using a boat or from a bridge. Samples were collected at three locations on the Pandu River and upstream and downstream of the confluence point of Pandu with the Ganges River. Pandu River sampling was done from the Pandu Bridge (26° 07′ 37.2″ N, 80° 38′ 45.1″ E, altitude 93 ± 3 masl), which is approximately 5 km upstream of the confluence with the Ganges River. The Ganges River was sampled upstream (26° 07′ 20.8″ N, 80° 39′ 48.1″ E) and downstream (latitude 26° 07′ 14.2″ N and longitude 80° 39′49.6″ E) of the Pandu River inflow. Samples were collected approximately monthly between February 2015 and April 2016. We would like to highlight that water level of Pandu River is controlled primarily by base flow, runoff from agricultural fields, and rain-events. Micrometeorological data obtained from a weather station (latitude, 26° 30′ 47.69″ N; longitude, 80° 13′ 56.39″ E) installed close to our sampling site revealed that rain events in 2015 were mostly restricted to the months of July, August, and September. For example, July, August, and September had 12, 4, and 1 days, C

DOI: 10.1021/acsearthspacechem.7b00112 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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ACS Earth and Space Chemistry the total flux of each nutrient was divided by the drainage area of the river.

other tropical rivers of India (Table 3). The only three Indian rivers that have higher DIN yields when compared to Pandu River were Haldia, Netravati, and Zuari.42 It is noteworthy to mention that nitrate yield in the Pandu River (0.162 t/km2/yr, Table 2) became one of the lowest when compared to some of the world rivers56 such as Yangtze (1.736 t/km2/yr), Trinity (1.178 t/km2/yr), Amazon (0.797 t/km2/yr), and Mississippi (1.302 t/km2/yr). The nitrate yield for Pandu River was only higher than some of the Arctic Rivers35 such as Ob (0.47 t/ km2/yr), Yenisey (0.38 t/km2/yr), Yukon (0.085 t/km2/yr), and Mackenzie (0.02 t/km2/yr). On the contrary, the phosphate yield for Pandu River is considerably higher (0.12 t/km2/yr, Table 2) when compared with rivers like Amazon (0.009 t/km2/yr),56 Yangtze (0.027 t/km2/yr),56 and Ganges (0.04 t/km2/yr, Table 2). It is noteworthy to mention that agricultural management practices, waste management, and effluent discharge policies, basin wide socio-economic conditions, cropping intensities, and cultivation patterns in the Indo-Gangetic flood plains are unique and direct comparison with other river systems around the world27−29,34−38 may not be straightforward. Flux of DIN and phosphate was highest during the monsoon season since the Pandu River is primarily rain-fed (Figure 4). During the monsoon season each year, rainfed rivers in southern India export (0.12 ± 0.03) × 106 t/yr of DIN and (0.08 ± 0.02) × 106 t/yr of phosphate to the Bay of Bengal (Krishna et al.).43 The glacial-fed Indian rivers in northern India have much higher concentrations of N and P due to their larger catchment area and volume of discharge; furthermore, fertilizer application in northern India (∼18 × 106 t/yr) is ∼5 times higher than southern India (3.5 × 106 t/yr).57 It is estimated that the glacial-fed rivers draining the Indian subcontinent (excluding Irrawaddy and Salween) export (1.62 ± 0.40) × 106, (0.27 ± 0.06) × 106 and (3.58 ± 0.89) × 106 t/ yr of nitrate, phosphate, and silicate, respectively to the northern Indian Ocean (Krishna et al. 2016).43 Our estimates show that the Pandu River exports 793 ± 128 t/yr of DIN and 177 ± 29 t/yr of phosphate to the Ganges River (Table 2), which accounts for 0.1% and 0.42% of total DIN and phosphate fluxes respectively to the Bay of Bengal (Table 3). Given that the Pandu River only accounts 0.03% of the total Ganges water discharge to Bay of Bengal, the Pandu River clearly carries a disproportionately high N and P load. Models such as Global Nutrient Export from Watersheds (NEWS) 58 and the SENEQUE program based on the Riverstrahler Model59 could not be applied since many input parameters such as potential evapotranspiration, fluxes of suspended matter, inputs of urban and industrial wastewater, and basin morphology have not been characterized. 3.1. Importance of Small Floodplain Rivers to Ganges Nutrient Budget. Since fertilizer application, industrial, and municipal wastes are the primary inputs of N and P to rivers in the Indo-Gangetic floodplain, population density and fertilizer application rates are appropriate metrics for assessing the representativeness of the Pandu River. To scale up the results obtained from Pandu to other rain-fed rivers in the IndoGangetic plain rivers, we therefore compared the consumption of fertilizer per square kilometer of irrigated land area for all the districts (Farrukhabaad, Kannuj, Auraiya, Kanpur Dehat, Kanpur Nagar and Fatehpur) within Pandu basin with 118 districts over the Indo-Gangetic plains (Table S2; Supporting Information). These 118 districts cover 89% of the total length of the River Ganges. Compilation of data from various governmental agencies (Table S2; Supporting Information)

3. RESULTS AND DISCUSSIONS The DIN, phosphate, and silicate concentrations in Pandu River ranged from 0.6−7.23, 0.08−4.58. and 6.99−21.48 mg/L, respectively, which is higher than other monsoonal rivers of India (Figure 3, Table 1). The ammonium, nitrite, nitrate, phosphate, and silicate yields of the Pandu River were 0.248, 0.12, 0.162, 0.118, and 1.08 t/km2/yr, respectively (Table 2). Pandu River has also a higher nutrient yield when compared to

Figure 3. Yields, fluxes, and concentrations of DIN, PO43−, and SiO2 in rain-fed rivers draining into Bay of Bengal.43 Data on Pandu River are from this study. D

DOI: 10.1021/acsearthspacechem.7b00112 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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ACS Earth and Space Chemistry Table 1. Time Series Measurements of Concentrations of Nutrientsa location name Pandu Pandu Pandu Pandu Pandu Pandu Pandu Pandu Pandu Pandu Pandu Pandu Ganga Ganga Ganga Ganga Ganga Ganga Ganga Ganga Ganga Ganga Ganga Ganga Ganga Ganga Ganga Ganga Ganga Ganga Ganga Ganga Ganga Ganga Ganga

River River River River River River River River River River River River U/C U/C U/C U/C U/C U/C U/C U/C U/C U/C U/C U/C D/C D/C D/C D/C D/C D/C D/C D/C D/C D/C D/C

collection date

sample ID

21-02-2015 20-03-2015 16-04-2015 25-06-2015 08-07-2015 02-09-2015 07-10-2015 15-12-2015 31-01-2016 19-02-2016 11-03-2016 08-04-2016 21-02-2015 20-03-2015 16-04-2015 25-06-2015 08-07-2015 02-09-2015 07-10-2015 15-12-2015 31-01-2016 19-02-2016 11-03-2016 08-04-2016 20/03/2015 16/04/2015 25/06/2015 08/07/2015 02/09/2015 07/10/2015 15/12/2015 31/01/2016 19/02/2016 11/03/2016 08/04/2016

S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S

nitrate (mg/L)

nitrite (mg/L)

ammonium (mg/L)

phosphate (mg/L)

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.09 ± 0.02 0.73 ± 0.07 0.45 ± 0.08 1.16 ± 0.09* 0.36 ± 0.04 0.59 ± 0.001 1.26 ± 0.02 0.33 ± 0.01 0.62 ± 0.09* 0.05 ± 0.002* 0.01 ± 0.001* 1.27 ± 0.09* 0.26 ± 0.03 0.27 ± 0.06 0.24 ± 0.01 0.19 ± 0.02 0.22 ± 0.04 0.08 ± 0.01 0.08 ± 0.02 0.47 ± 0.04 0.44 ± 0.03* 0.33 ± 0.001* 0.49 ± 0.02* 0.41 ± 0.04* 0.86 ± 0.18 0.19 ± 0.02 0.2 ± 0.03 0.33 ± 0.03 0.65 ± 0.02 0.08 ± 0.003* 0.33 ± 0.001* 0.57 ± 0.02* 0.33 ± 0.004* 0.46 ± 0.02* 0.57 ± 0.06*

2.55 ± 0.02 2.32 ± 0.07 2.47 ± 0.11 2.56 ± 0.25 0.56 ± 0.01 2.24 ± 0.05 2.5 ± 0.08 2.4 ± 0.08 1.33 ± 0.03 0.27 ± 0.01* 0.03 ± 0.005* 2.64 ± 0.06 0.7 ± 0.34 0.02 ± 0.001 0.01 ± 0.012 0.02 ± 0.01 0.25 ± 0.02 0.04 ± 0.02 0.04 ± 0.01 1.87 ± 0.23* 1.08 ± 0.04 0.62 ± 0.05* 1.46 ± 0.05 0.88 ± 0.05* 1.57 ± 0.60 0.01 ± 0.007 0.04 ± 0.02 0.4 ± 0.02 1.49 ± 0.26 0.04 ± 0.01 0.85 ± 0.01 0.94 ± 0.04* 0.2 ± 0.01 1.61 ± 0.04 1.88 ± 0.25*

2.92 ± 0.43* 1.05 ± 0.65 2.72 ± 0.16 3.38 ± 0.88 0.08 ± 0.01 0.53 ± 0.04 2.27 ± 0.01 1.24 ± 0.09* 0.31 ± 0.05* 0.78 ± 0.02* 1.69 ± 0.01* 4.58 ± 0.21 0.08 ± 0.03 0.02 ± 0.02 0.03 ± 0.01 0.05 ± 0.001 0.06 ± 0.003 0.03 ± 0.01 0.04 ± 0.01 0.1 ± 0.01* 0.07 ± 0.02* 0.22 ± 0.03* 0.22 ± 0.04* 0.12 ± 0.01 0.56 ± 0.01 0.02 ± 0.01 0.09 ± 0.01 0.06 ± 0.01 0.28 ± 0.01 0.05 ± 0.22 0.03 ± 0.004 0.08 ± 0.01* 0.27 ± 0.01* 0.2 ± 0.05 0.39 ± 0.02

1/1 2/4 3/1 4/3 5/2 6/3 7/3 8/13 9/4 10/1 11/1 12/1 1/3 2/2 3/3 4/1 5/4 6/1 7/2 8/11 9/2 10/3 11/3 12/2 2/1 3/4 4/4 5/3 6/4 7/4 8/10 9/3 10/4 11/2 12/3

0.63 0.84 0.91 0.95 1.2 1.23 1.76 0.82 1.64 0.36 0.56 3.32 0.48 0.88 0.97 0.62 0.86 1.13 0.8 1.03 0.91 0.5 0.23 0.65 0.92 1.04 0.64 0.81 1.11 0.92 0.64 1.14 0.68 0.30 0.75

0.09 0.13 0.14 0.14 0.18 0.19 0.27 0.12 0.25 0.05 0.08 0.50 0.07 0.13 0.15 0.09 0.13 0.17 0.12 0.16 0.14 0.08 0.04 0.10 0.14 0.16 0.10 0.12 0.17 0.14 0.10 0.17 0.10 0.05 0.11

silicate (mg/L) 14.24 9.12 12.72 15.08 8.89 7.93 17.7 6.99 14.32 19.64 20.33 21.48 1.79 8.32 7.36 0.17 9.3 7.66 6.77 8.44 0.78 0.72 2.62 7.86 7.85 7.93 0.32 7.28 8.82 4.62 8.57 0.82 0.72 2.61 8.36

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.97 0.71 2.35 0.17 0.01 0.01 0.70 1.23 1.62 0.02 0.15 2.10 0.95 0.01 0.01 0.01 0.01 0.01 3.01 2.22 0.03 0.04 0.23 0.79 4.66 0.01 0.01 0.01 0.01 0.01 0.84 0.01 0.04 0.08 0.61

a

Concentrations of nutrients in Pandu River, Ganges River before it confluences with Pandu River (U/C stands for upstream confluence) and Ganges River after it confluences with Pandu River (D/C stands for downstream confluence). Samples with * were analyzed in triplicates. All others are duplicate analysis.

Table 2. Monthly Average Nutrient Fluxes for the Pandu Rivera months Feb, 2015 Mar, 2015 Apr, 2015 Jun, 2015 Aug, 2015 Sep, 2015 Oct, 2015 Dec, 2015 Jan, 2016 Feb, 2016 Mar, 2016 Apr, 2016 annual flux (t/yr) yield (t/km2/yr) a

silicate (t/month) 43.50 29.98 66.20 46.22 294.56 320.90 479.98 36.25 52.32 50.68 58.67 140.17 1619 1.08

± ± ± ± ± ± ± ± ± ± ± ± ±

6.80 5.37 11.25 8.78 52.07 52.62 86.47 6.79 7.73 7.09 8.36 30.19 120

nitrite (t/month) 0.81 0.66 7.02 5.29 37.03 37.03 81.64 0.99 1.19 0.66 0.81 7.02 180 0.12

± ± ± ± ± ± ± ± ± ± ± ± ±

0.41 0.36 4.73 3.14 26.53 26.53 56.48 0.48 0.55 0.36 0.41 4.73 68

nitrate (t/month) 2.50 2.28 7.46 6.35 63.55 63.55 78.87 2.74 2.98 2.28 2.50 7.46 243 0.162

± ± ± ± ± ± ± ± ± ± ± ± ±

0.43 0.43 1.79 1.32 16.36 16.36 19.53 0.44 0.46 0.43 0.43 1.79 30

phosphate (t/month) 4.38 3.74 23.38 18.93 19.83 19.83 43.50 5.08 5.84 3.74 4.38 23.88 177 0.118

± ± ± ± ± ± ± ± ± ± ± ± ±

1.40 1.29 10.38 7.15 9.20 9.20 19.45 1.52 1.67 1.29 1.40 10.38 29

ammonium (t/month) 16.12 3.19 13.69 23.93 47.94 94.08 149.65 11.13 3.69 0.72 0.54 5.86 371 0.248

± ± ± ± ± ± ± ± ± ± ± ± ±

10.57 1.56 7.79 12.97 24.78 43.86 88.82 6.67 1.60 0.34 0.26 3.94 104

DIN (t/month) 19.43 6.13 28.16 35.57 148.51 194.66 310.16 14.85 7.86 3.65 3.85 20.33 793 0.53

± ± ± ± ± ± ± ± ± ± ± ± ±

10.59 1.66 9.29 13.41 39.82 53.81 107.05 6.75 1.76 0.66 0.66 6.41 128

Yield of Pandu River was obtained by dividing total flux of each nutrient by the drainage area of the river (1495 km2).

shows that fertilizer consumed per square kilometer of irrigated land area in the year 2010−2011 in Pandu basin was similar to

the rest of the basin. For example, the states of Uttar Pradesh (70 districts), Bihar (38 districts), and West Bengal (16 E

DOI: 10.1021/acsearthspacechem.7b00112 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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Table 3. Catchment Area, Nutrient Flux, and Nutrient Yield of Pandu River in Comparison to Other Tropical Rivers in India43 river Haldia Subernarekha Baitarani Mahanadi Rushikulya Vamsadhara Nagavali Godavari Krishna Penna Ponnayaar Vellar Cauvery Ambalyaar Vaigai Narmada Tapti Sabarmati Mahisagar Kochi backwaters Chalakudi Bharathappuzha Netravati Sharavati Khali Zuari Mandovi Pandua Gangesb

catchment area 106 km2 0.010 0.019 0.014 0.142 0.009 0.011 0.009 0.313 0.259 0.055 0.016 0.009 0.088 0.007 0.099 0.065 0.022 0.026 0.002 0.006 0.003 0.004 0.004 0.001 0.004 0.001495 0.980

DIN flux t/yr

phosphate flux t/yr

silicate flux t/yr

DIN yield t/km2/yr

phosphate yield t/km2/yr

silicate yield t/km2/yr

14227 1250 4744 7800 146 332 137 79571 9321 394 153 65 1602 107 69 48355 14973 3713 12856 5843 592 589 7863 1820 1158 1352 903 793 797752

20960 303 2384 7071 66 149 83 13192 21180 1134 165 234 8008 156 69 4300 6183 603 4236 3284 388 1034 2400 1151 1482 1367 3963 177 42467

37157 9761 28864 108439 865 2342 786 223764 362876 36186 12096 3531 74725 2566 995 48042 15546 2184 14749 11932 2454 5964 7504 3614 4151 2154 2938 1619

1.40 0.07 0.33 0.06 0.02 0.03 0.02 0.25 0.04 0.01 0.01 0.01 0.02

2.06 0.02 0.17 0.05 0.01 0.01 0.01 0.04 0.08 0.02 0.01 0.03 0.09

3.64 0.51 2.03 0.77 0.10 0.21 0.08 0.72 1.40 0.66 0.76 0.41 0.85

0.01 0.49 0.23 0.17 0.50

0.01 0.04 0.10 0.03 0.17

0.14 0.49 0.24 0.10 0.58

0.35 0.10 2.46 0.51 0.28 1.35 0.25 0.53 0.81

0.23 0.17 0.75 0.32 0.35 1.37 1.10 0.12 0.04

1.44 0.96 2.35 1.00 0.99 2.15 0.82 1.08

a

Values are taken from Table 2. bNutrient transport by Ganges to the ocean was calculated by multiplying the annual discharge value of Ganges (4.9 × 1011 m3/yr) with the average nutrient concentration data reported in Table 1. Nitrite flux to the ocean was 142 × 103 t/yr. Drainage area and annual discharge values are taken from Milliman, 2011.63

Figure 4. Monthly variations in river discharge (orange curve) rainfall (black curve),62 DIN flux (green curve), and phosphate flux (blue curve) in the Pandu River between February 2015 and April 2016.

(Table S2, Supporting Information,).60,61 In addition, population densities for 2011 were 9 ± 4, 11 ± 3, 13 ± 8, and 8 ± 3 for Uttar Pradesh, Bihar, West Bengal, and Pandu basin,

districts) have an average annual (2010−2011) fertilizer consumption of 34 ± 14, 44 ± 29, and 50 ± 16 t/km2, respectively, which is similar to Pandu basin ∼47 ± 17 t/km2 F

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ACS Earth and Space Chemistry Notes

respectively, were also similar (Table S2; Supporting Information). Because the population density and average annual fertilizer consumption per square kilometer of irrigated land are comparable, we conclude data from Pandu River can be scaled up to understand the cumulative impact of all small floodplain rivers in the Indo-Gangetic floodplains to Ganges river system. Pandu River will therefore act as a fair representative of all small flood plain rivers in the IndoGangetic plains. The upscaling calculation revealed that these small rivers would annually export 1,15,903 and 25,706 t/yr of DIN and phosphate to the Ganges River. Since the Ganges River annually exports 7,97,752 and 42,467 t of DIN and phosphate to Bay of Bengal (Table 3), the small flood plain rivers collectively contributes 15% and 61%, respectively, of the total DIN and phosphate fluxes of the Ganges River to the Bay of Bengal. Since an extensive nutrient data set collected over several years and from several smaller rivers does not exist, the numbers will have uncertainties, but certainly the estimated numbers can be treated as a first order estimation.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS I.S.S. acknowledges financial support from USAID PEER Grant AID-OAA-A-11-00012 and IIT Kanpur Initiation Grant 2013389. S.R. is thankful for the M.Tech. scholarship at Indian Institute of Technology Kanpur. Sincere thanks to Bernhard Peucker-Ehrenbrink and Britta Voss for their comments and suggestions on the manuscript.

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4. CONCLUSIONS This study quantifies the importance of small floodplain rivers in exporting DIN and phosphate to a large river system. The study shows that Pandu, a small Indo-Gangetic floodplain river annually delivers 793 ± 128 tonnes of DIN and 177 ± 29 tonnes phosphate to the Ganges. Cumulatively, the small floodplain rivers in the Indo-Gangetic floodplain could contribute ∼15% and ∼61% of the DIN and phosphate fluxes that Ganges River delivers into Bay of Bengal. Therefore, small floodplain rivers will act as an important nonpoint pollution source of nutrients to large river systems. This knowledge will be critical to act on the nutrient remediation efforts in IndoGangetic plains and will highlight the remediation challenges posed by these small flood plain rivers. Despite the importance of small flood plain rivers to river nutrient budgets, these rivers are rarely studied or monitored. Our study therefore provides a framework that demonstrates the need for additional assessments of N and P transport via smaller floodplain rivers. We assert that quantifying the contributions from smaller floodplain rivers in the Indo Gangetic plains and further monitoring them over longer periods will help local managers to better restore the health of Ganges River and adapt to future impacts.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsearthspacechem.7b00112. Contains data to compare Pandu river basin with the rest of the Ganges basin in terms of geographical area, population density, irrigated land area, and annual fertilizer consumption rates. To assess the data quality, measured nutrient concentrations are compared with the in-house standards are also presented (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Indra S. Sen: 0000-0001-7302-2313 G

DOI: 10.1021/acsearthspacechem.7b00112 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acsearthspacechem.7b00112 ACS Earth Space Chem. XXXX, XXX, XXX−XXX