Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)
Article
Tracking nitrogen sources, transformation and transport at a basin scale with complex plain river networks Qitao Yi, Qiuwen Chen, Liuming Hu, and Wenqing Shi Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 20 Apr 2017 Downloaded from http://pubs.acs.org on April 20, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 28
Environmental Science & Technology
1
Tracking nitrogen sources, transformation and transport at a basin scale with complex
2
plain river networks
3
Qitao Yi2,3, Qiuwen Chen1,2*, Liuming Hu1, Wenqing Shi1
4
1
5
210098, China
6
2
7
100085, China
8
3
9
232001, China
Center for Eco-Environment Research, Nanjing Hydraulic Research Institute, Nanjing
Research Center for Eco-Environment Sciences, Chinese Academy of Sciences, Beijing
School of Earth and Environment, Anhui University of Science and Technology, Huainan
10
*
11
E-mail:
[email protected] Corresponding author: Hujuguan 34, Nanjing 210098, China. Tel./Fax: +86 25 85829765,
1
ACS Paragon Plus Environment
Environmental Science & Technology
12
Table of contents
13
2
ACS Paragon Plus Environment
Page 2 of 28
Page 3 of 28
Environmental Science & Technology
14
ABSTRACT: This research developed an innovative approach to reveal nitrogen sources,
15
transformation and transport in large and complex river networks in the Taihu Lake basin
16
using measurement of dual stable isotopes of nitrate. The spatial patterns of δ15N
17
corresponded to the urbanization level, and the nitrogen cycle was associated with the
18
hydrological regime at the basin level. During the high flow season of summer, nonpoint
19
sources from fertilizer/soils and atmospheric deposition constituted the highest proportion of
20
the total nitrogen load. The point sources from sewage/manure, with high ammonium
21
concentrations and high δ15N and δ18O contents in the form of nitrate, accounted for the
22
largest inputs among all sources during the low flow season of winter. Hot spot areas with
23
heavy point source pollution were identified, and the pollutant transport routes were revealed.
24
Nitrification occurred widely during the warm seasons, with decreased δ18O values; whereas
25
great potential for denitrification existed during the low flow seasons of autumn and spring.
26
The study showed that point source reduction could have effects over the short term; however,
27
long-term efforts to substantially control agriculture nonpoint sources are essential to
28
eutrophication alleviation for the receiving lake, which clarifies the relationship between
29
point and non-point source control.
3
ACS Paragon Plus Environment
Environmental Science & Technology
30
INTRODUCTION
31
Over the past century, surplus nitrogen (N) has been loaded into the biosphere due to intensive
32
anthropogenic activities, resulting in excessive nitrogen in surface water and contributing to water
33
quality impairment, eutrophication and ecological disasters.1,2 Reduction of extra nitrogen load is
34
the fundamental way to improve water quality and restore aquatic ecosystems. Strong efforts to
35
identify nitrogen pollution sources entering into rivers and lakes have been attempted in the past
36
several decades.2-4 Nitrogen sources are classified into point sources and nonpoint sources.
37
Traditionally, point sources come from domestic sewage, industry discharge and livestock manure,
38
and nonpoint sources come from agricultural fertilizer, soil erosion, and atmospheric dry and wet
39
deposition. There are many methods for identifying nitrogen sources and load from a watershed to
40
the receiving waters. In general, the point source pollution load can be obtained through detailed
41
statistics, whereas the nonpoint source pollution load from a catchment is estimated by watershed
42
models.5-7
43
Despite great efforts to identify point source or nonpoint source pollution related to different land
44
use,8,9 nitrogen source identification remains challenging in urbanized and industrialized areas with
45
complex land use. The problem is intensified where densely crisscrossed river networks are
46
concerned; complicated flow patterns make nitrogen transport processes difficult to trace.10
47
Although some researchers correlate nitrogen in waters with land use using statistical
48
approaches,11,12 the results are qualitative with great uncertainty. The relationship between complex
49
source loading and water quality in plain river networks is not sufficient to support basin-scale
50
nitrogen management. Consequently, many lakes suffer high external nutrient inputs and algal
51
blooms.13,14
52
The dual isotope (combination of 15N and 18O in nitrate) approach has been applied to trace sources 4
ACS Paragon Plus Environment
Page 4 of 28
Page 5 of 28
Environmental Science & Technology
53
of nitrate and their potential transformation from atmospheric deposition, soils, chemical fertilizers,
54
and sewage or manure, both in surface and ground water.2,15-22 The application fields cover different
55
types of land use, including forested,15,16 agricultural,17 urbanized and hybrid areas.18-22 During
56
recent years, the analytical methodologies for both δ15N and δ18O have improved considerably, and
57
have become an attractive technique to identify nitrogen sources in surface or groundwater.23,24
58
However, adapting the method to identify nitrogen sources in complex plain river networks, which
59
have variable
60
remains highly ambitious.
61
The main objectives of this work are to: (1) develop a comprehensive approach by combining
62
analysis of water quality and dual isotopes of nitrate to identify the dominant sources and
63
transformation processes of nitrogen in complex river networks of a lake basin; (2) reveal spatial
64
distribution patterns and transport routes of nitrate in complex river networks with relation to land
65
use and hydrological regime; (3) quantify the potential for reduction of nitrogen, and evaluate the
66
effectiveness of nutrient management strategy at a basin scale for alleviation of lake eutrophication.
67
MATERIALS AND METHODS
68
Study area. Lake Taihu is the third largest freshwater lake in China. It covers an area of 2338 km2
69
with an average depth of 1.9 meters and a corresponding volume of 4.4 billion m3. The Taihu basin
70
has an area of approximately 36,895 km2 and is located in the downstream area of the Yangtze
71
River (Figure 1). The basin is heavily populated and highly industrialized, with only 0.4% of
72
China’s land area supporting 40 million residents and 11% of the Gross Domestic Product of the
73
country. The lake suffers serious eutrophication and cyanobacterial blooms due to excessive
74
external nutrient loading from the Taihu basin. The intensified land-use of industrialization,
75
urbanization and agriculture in the basin has produced high nitrogen loading from multiple
spatiotemporal nitrogen loading patterns, from an integrated basin-scale perspective
5
ACS Paragon Plus Environment
Environmental Science & Technology
76
sources.10,12 Consequently, two-thirds of the lake area exceeds the level of 1.0 mg L-1 total nitrogen
77
and 0.05 mg L-1 total phosphorus, the nutrient concentration thresholds for controlling
78
eutrophication in Lake Taihu required by the government.
79
The basin is divided into eight parts in terms of hydraulic characteristics (Figure 1b), and the river
80
network is well developed, consisting of over 200 main rivers crisscrossing the basin. This study
81
focuses on the upstream areas in the northwest of the Taihu basin, covering the whole area of the
82
Huxi part and half the area of the Wu-Xi-Chen part (Figure 1b). These two parts are the most
83
heavily polluted areas and account for over 70% of the pollution loads entering into the lake. The
84
west and south are the upstream rivers along with mountains, and the north is bounded by the
85
Yangtze River. Plain river networks characterize the hydraulics of the study areas, where west-east
86
rivers crisscross with north-south rivers. Specific information on Taihu basin climate, hydrology
87
and river networks in the study area can be found in the Supporting Information or literature.10
88 89
Figure 1. Location of study areas (a, b), and sampling sites (c) in the upstream river network of Lake Taihu. (Note:
90
the river network is simplified from more complex rivers; “Route Two” and “Route Three” refer to the Water
91
Transfer Projects between the Yangtze River and Lake Taihu, and the arrows indicate water transfer directions.)
92
Sampling design. Forty-eight sampling sites are located in the main rivers (Figure 1c). Specific
93
information concerning sampling sites is listed in Table S1 (Supporting Information). Three 6
ACS Paragon Plus Environment
Page 6 of 28
Page 7 of 28
Environmental Science & Technology
94
sampling campaigns were conducted in the high flow season of summer, low flow season of autumn
95
and low flow season of winter in late June 2015, October 2015 and January 2016, respectively.
96
Monthly samplings from June 2015 to April 2016 were conducted in the thirteen main inflowing
97
rivers to obtain more details on the nitrogen loading patterns. Water samples were collected at the
98
Yangtze River sites in June of 2015 and January of 2016. The details of the hydrological regime of
99
the study area are shown in Figure S1 in Supporting Information. Water volume at Lake Taihu
100
reached its peak in the heavily rainy June of 2015, declined towards the low flow of autumn and
101
winter, and increased with rainfall events in the spring of 2016.
102
Water quality and stable isotope analysis. Surface water samples were taken at 0.5–1.0 m depth
103
under the surface using a 5 L Plexiglas water sampler. The main analyzed parameters included
104
water temperature, pH, dissolved oxygen, electricity conductivity, total nitrogen, total dissolved
105
nitrogen, dissolved inorganic nitrogen forms of nitrate, nitrite and ammonium, and chloride. Details
106
on sampling methods and water quality analysis are provided in Supporting Information.
107
Collected river samples for stable isotopic analysis were filtered with 0.2-µm cellulose ester filters
108
and frozen below -20oC until analysis. The denitrifier method at the Environmental Stable Isotope
109
Lab in Chinese Academy of Agricultural Sciences (CAAS), Beijing was used for analyzing δ15N
110
and δ18O in nitrate. Briefly, denitrifying bacteria (Pseudomonas auroeofaciens) convert nitrate to
111
gaseous
112
(Tracegas-Isoprime100, Germany). Isotopic ratio values are reported in parts per thousand (‰)
113
relative to atmospheric N2 and Vienna Standard Mean Ocean water (VSMOW) for δ 15N and δ18O,
114
respectively:
115
∆sample (‰)=[(Rsample-Rstandard)/Rstandard] ×1000
116
where ∆sample is the stable isotope ratio in the samples, Rsample is the ratio of 15N/14N or 18O/16O in the
nitrous
oxide
(N2O),
detected
using
an
isotope
7
ACS Paragon Plus Environment
ratio
mass
spectrometer
Environmental Science & Technology
117
samples, Rstandard is the ratio of 15N/14N or 18O/16O in the standards. Sample analysis had an average
118
precision of 0.2‰ for δ15N-nitrate and 0.7‰ for δ18O-nitrate.
119
The land use of 2010, combined with the hydrological regime (Figure S1 and Table S2 in
120
Supporting Information), of the study area was used to analyze their effects on spatial patterns of
121
dual isotopes of nitrate. Sampling sites at the downstream of the flow direction were selected for
122
statistical analysis. Pearson correlation coefficients, at confidence levels of 95% (p
