Dynamics and sources of colloids in shallow groundwater in lowland wells and fracture flow in sloping farmland

Accepted Manuscript Dynamics and sources of colloids in shallow groundwater in lowland wells and fracture flow in sloping farmland Wei Zhang, Jian-Hua Cheng, Qing-Song Xian, Jun-Fang Cui, Xiang-Yu Tang, Gen-Xu Wang PII:

S0043-1354(19)30225-8

DOI:

https://doi.org/10.1016/j.watres.2019.03.012

Reference:

WR 14498

To appear in:

Water Research

Received Date: 16 May 2018 Revised Date:

4 March 2019

Accepted Date: 12 March 2019

Please cite this article as: Zhang, W., Cheng, J.-H., Xian, Q.-S., Cui, J.-F., Tang, X.-Y., Wang, G.-X., Dynamics and sources of colloids in shallow groundwater in lowland wells and fracture flow in sloping farmland, Water Research (2019), doi: https://doi.org/10.1016/j.watres.2019.03.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Dynamics and sources of colloids in shallow groundwater in

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lowland wells and fracture flow in sloping farmland

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Wei Zhanga,b,c, Jian-Hua Chengb,c, Qing-Song Xianb,c, Jun-Fang Cuib,c,*, Xiang-Yu

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Tangb,c, Gen-Xu Wangb,c

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b

Key Laboratory of Mountain Surface Processes and Ecological Regulation, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu

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610041, China

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University, Chongqing 400067, China

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School of Tourism and Land Resource, Chongqing Technology and Business

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University of Chinese Academy of Sciences, Beijing 100049, China

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* Corresponding author: Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, No.

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9, Block 4, Renminnanlu Road, Chengdu 610041, China. Tel.: +86 28 85213556; fax: +86 28 85222258.

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E-mail address: [email protected] (J.F. Cui).

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Abstract

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Field-scale studies of natural colloid mobilization and transport in finely fractured aquifer as well

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as the source identification of groundwater colloids are of great importance to the safety of

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shallow groundwater. In this study, the daily monitoring of fracture flow from a sloping farmland 1

ACCEPTED MANUSCRIPT plot and the biweekly monitoring of three lowland shallow wells within the same catchment were

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carried out simultaneously in 2013. The effects of physicochemical perturbations on groundwater

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colloid dynamics were explored in detail using partial redundancy analysis, structural equation

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modeling, Pearson correlation and multi-linear regression analyses. The characterization and

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source identification of groundwater colloids were addressed via multiple parameters. The daily

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colloid concentration in the fracture flow varied between 0.54 and 31.90 mg/L (1.64 mg/L on

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average). Unique periods of high colloid concentration (5.59 mg/L on average) occurred during

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the initially generated flow following the dry season. In comparison, a narrower colloid

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concentration range of 0.24-11.66 mg/L was observed in the lowland shallow wells, with a smaller

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temporal variation than that of the fracture flow. A low percentage (2.4-7.0%) of colloids and a

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high percentage (47.7-92.0%) of coarse particles (2-10 µm) were present in the lowland well water.

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Hydraulic perturbation by rainwater infiltration in the sloping farmland was the dominant

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mechanism for colloid mobilization in general; this effect retreated to secondary importance

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behind chemical perturbations (pH, Mg2+ and DOC) at low flow discharges (<1.3 L/min). In

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contrast, water chemistry (e.g., EC, cations and DOC concentrations) exhibited a major effect on

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colloid dynamics in the water of the lowland wells, except for the extremely high-salinity water of

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one well, in which water temperature showed a negative dominant influence on colloid stability.

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The combined use of multiple parameters (e.g., mineral composition and organic matter, calcium

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carbonate and δ13C contents) traced groundwater colloids to the shallow soil in the upper

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ACCEPTED MANUSCRIPT farmlands. It is strongly advised that in finely fractured aquifers within agricultural catchments,

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not only the small colloids but also the coarse particles in the size range of 2-10 µm should be

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monitored in case of colloid-associated contamination from agricultural wastes e.g., N, P,

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pesticides and/or heavy metals, especially at the early stages of the rainy seasons.

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Keywords: Fracture flow, groundwater, colloid transport, source identification

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1. Introduction

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Mass transfer in the vadose zone has been a major concern in transport studies of

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different geospheres in general. In particular, the frequent temporal and spatial

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variations in physical and chemical conditions in the vadose zone exert a great effect

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on the fate (e.g., release, mobilization and transport) of water dispersible colloids.

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Colloids include fine particles of several micrometers to macromolecules of several

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nanometers and are ubiquitous in the subsurface (Liu et al., 2018). The release of

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colloids from a solid surface is determined mechanically by the interactions of

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repulsive and attractive forces between the colloid and the solid surface. Only when

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the repulsive forces overcome the attractive forces can colloids be detached and

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mobilized (Degueldre and Benedicto, 2012; Torkzaban et al., 2010; Zhang et al.,

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2012). Mobilized colloids are susceptible to transport into groundwater, during which

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colloid-facilitated transport of various strongly sorbing contaminants (e.g.,

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ACCEPTED MANUSCRIPT phosphorus, nitrogen, pharmaceuticals, pesticides, heavy metals and radionuclides)

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has been frequently reported (Delwiche et al., 2014; Ge et al., 2018; Judy et al., 2018;

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Missong et al., 2018; Mohanty et al., 2014; Tran et al., 2018; Xing et al., 2015). In

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particular, pathogens, e.g., viruses and bacteria, either occur in groundwater in bio-

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colloid forms (Zhang et al., 2012) as in most cases, or are associated with inorganic

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colloids in wastewater (Walsge et al., 2010). Both forms have been demonstrated to

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move through shallow aquifers with little retention and are present in groundwater

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due to colloid-facilitated transport (Göppert and Goldscheider, 2008; Maciopinto et al.,

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2008), which may lead to the outbreak of waterborne diseases. Therefore, it is of great

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importance to explore subsurface colloid transport characteristics, which provides a

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pre-requisite for experimental and modeling research on colloid-facilitated

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contaminant transport.

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Numerous studies concerning colloid mobilization and subsequent transport, as well

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as the facilitated transport of various contaminants by colloids, have been carried out

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in the laboratory, e.g., in repacked/intact columns and fractured cores (Ge et al., 2018;

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Kuno et al., 2002; Liu et al., 2018; Mishurov et al., 2008; Mitropoulou et al., 2013;

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Mohanty et al., 2014; Mondal and Sleep, 2012; Morales et al., 2011; Tran et al., 2018;

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Walshe et al., 2010). The flow rate, pH, ionic strength, divalent cations, dissolved

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organic matter, etc., of a solution have been widely reported to affect colloid

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ACCEPTED MANUSCRIPT mobilization, transport and the association of contaminants to colloids in these studies.

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However, the ideal or synthetic colloid tracers (e.g., nanoparticles or microspheres) of

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specific sizes, shapes, densities and surface properties that were frequently used in

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these studies do not resemble the various natural colloids present in groundwater. In

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addition, colloid mobilization and transport in a column/core with a relatively

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homogeneous structure or a steady state of flow may exhibit large discrepancies from

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those observed in field scenarios. Seaman et al. (2007) demonstrated that colloid

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mobilization and transport in an aquifer did not follow shear force predictions from

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well-controlled column-scale studies. This discrepancy leads to the necessity for the

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field-scale study of the mobilization and transport of naturally occurring colloids in

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groundwater.

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At the field scale, water flow and organic carbon were observed to have a profound

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effect on colloid generation in groundwater (Kaplan et al., 1993). The released colloid

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load in shallow aquifers was also found to increase with the pH of recharge water,

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especially when it exceeded the pHpzc of the inorganic colloids (Rebacca et al. 2002).

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The authors also reported that the decrease in ionic strength did not facilitate

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significant colloid mobilization, which differs from that observed in many column-

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scale studies (e.g., Kuno et al., 2002; Mitropoulou et al., 2013; Mondal and Sleep,

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2012). Colloid mobilization and transport in response to natural rainfall and artificial

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ACCEPTED MANUSCRIPT irrigation events were also observed in vadose zone lysimeters and were partially

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attributed to the moving air-water interfaces in the unsaturated aquifer (Liu et al.,

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2013). A similar quick response of natural colloid mobilization and transport in

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fracture flow due to hydraulic scouring and the movement of the air-water interface

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has been demonstrated in a fractured mudstone (Zhang et al., 2015, 2016) and in a

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fractured karst aquifer (Pronk et al., 2009). In addition, the quick mobilization and

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transport of colloids within the first several hours following flow infiltration as well

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as the occurrences of lower colloid concentration peaks several days later in fractured

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chalk have been reported (Weisbrod et al., 2002). These results imply the potential for

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large-load and long-distance transport of colloids in groundwater originating from

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macroporous or fractured media where preferential flow pathways prevail. However,

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colloid mobilization in shallow aquifers, for example, induced by water injection,

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showed significant spatial and temporal heterogeneities due to the local hydrogeologic

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conditions (Seaman et al., 2007). Moreover, the complex interplay between physical

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perturbations of rainfall infiltrations and chemical reactions (e.g., water pH and ionic

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strength) contributed to colloid load in well water in karst aquifers (Shevenell and

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McCarthy, 2002). However, the relative importance of physical and chemical

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perturbation has not yet been identified.

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Nevertheless, the existing field-scale studies of natural colloid mobilization and

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ACCEPTED MANUSCRIPT transport in the subsurface were based on single events (e.g., natural or artificial

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rainfall, irrigation, groundwater recharge). The long tails and erratic pulses of colloid

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concentrations observed during the drainage process in these field-scale studies

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further indicate the importance of the investigation of colloid mobilization and

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transport over a longer time scale. In addition, the re-mobilization or re-suspension of

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the clogged/coagulated colloids and the increased load of small colloids caused by the

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breakdown of large aggregates may also occur due to the flow regime and the

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resultant variations in water chemistry induced over time, especially under fluctuating

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natural weather conditions (Göppert and Goldscheider, 2008). Continuous field-scale

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investigations of natural colloids and groundwater/flow dynamics in response to

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various rainfall events in different geological media are necessary to better understand

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the temporal and spatial variations of colloid transport in a large field. Various

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geological media (e.g., soils, weathered rocks, sediments, till deposits, epikarst) could

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act as the sources of natural colloids present in groundwater, but contribute different

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colloid loads (Albarran et al., 2014; Degueldre and Cloet, 2016; McCarthy and

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McKay, 2004; Pronk et al., 2009; Schiperski et al., 2016). In addition, field studies

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have indicated that colloid transport in response to water infiltration showed site-

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specific characteristics (e.g., breakthrough time, recovery, peak colloid concentration)

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(Göppert and Goldscheider, 2008; Zhang et al., 2016). Therefore, it is also important

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ACCEPTED MANUSCRIPT to trace the sources of natural colloids to more accurately evaluate the potential of

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colloid-facilitated contaminant transport. Particle size distribution (PSD) was used for

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the source identification of autochthonous particles inside an aquifer and

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allochthonous particles derived from the land surface in karst groundwater (Pronk et

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al., 2007). Morphological and mineralogical evidence is also useful for tracking the

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potential sources of suspended sediments in surface water (Chanudet and Filella, 2008;

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Mills et al., 2017) and shallow groundwater (Filella et al., 2009; Herman et al., 2007).

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However, the feasibility of these technologies for groundwater colloid source

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identification needs to be tested, and other technologies are still inadequate. 13C stable

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isotope analysis of organic matter provides information about carbon transfer between

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different geomedia with distinct carbon isotope signatures (Engelmann et al., 2018).

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This parameter has been increasingly applied in recent decades in the source

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identification of sediment from soils of different land uses within a catchment (Gibbs,

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2008; Guan et al., 2017). However, no application of

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identification has been conducted.

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The vast (160, 000 km2) hilly region of central Sichuan in the upper reaches of the

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Yangtze River is characterized by thin purple soil cover and underlying finely

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fractured mudrock. Frequent occurrences of preferential flow have been demonstrated

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in finely fractured aquifer in this region (Zhao et al., 2013); as a result, the mobilized

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C in colloid source

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ACCEPTED MANUSCRIPT colloids in the subsurface were enriched in the fracture flow (Zhang et al., 2015, 2016)

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and could be transported to lowland drinking wells.

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In this study, a sloping farmland plot (0.15 hm2) located in a relatively high part of the

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Jieliu catchment and three lowland shallow wells along the streamline to the

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catchment outlet were selected. Fracture flow from the hillslope and well water from

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the lowland wells were investigated simultaneously for a year to investigate the

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mechanisms regarding the mobilization and transport of natural colloids. Daily and

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biweekly monitoring were carried out for the fracture flow and the well water,

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respectively. The relationship between the dynamics of colloid concentration and the

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dynamics of physiochemical perturbations in the shallow groundwater was analyzed

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and discussed thoroughly using partial redundancy analysis (pRDA), structural

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equation modeling (SEM), Pearson correlation and multi-linear regression analysis. In

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addition, the characterization and source identification of groundwater colloids were

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carried out by multi-parameter comparisons between the colloids and nearby

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geological samples (soils and weathered mudrocks from the upper farmlands and/or

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forests). To the best of our knowledge, this is the first study to quantitatively

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characterize the interplay of physical and chemical factors in natural colloid dynamics

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for different shallow groundwater patterns (e.g., fracture flow and lowland well water)

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by a combination of pRDA and SEM. In addition, exploring the reliability and

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validity of coupling

C stable isotope with other chemical parameters (e.g., mineral

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composition, organic matter and calcium carbonate) to identify groundwater colloid

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sources is expected to provide new insights regarding colloid source identification.

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The in-situ investigation results presented in this study may provide theoretical and

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modeling information regarding natural colloid mobilization and transport in large

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fields (e.g., catchment-scale) and may also help protect shallow groundwater from

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potential contamination by colloids and colloid-associated agricultural wastes, in not

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only finely fractured media but also karst aquifers elsewhere.

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2. Materials and methods

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2.1 Experimental site

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The study site is located in the small Jieliu catchment (Fig. 1), which has an area of

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0.35 km2 and is located in hilly central Sichuan Province, Southwest China. The

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annual mean temperature and rainfall from 1981 to 2006 in this area were 17.3 °C and

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826 mm, respectively. The cumulative rainfall from May to September accounted for

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85% of the annual precipitation. Sloping farmland is the major land use type (a

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proportion of 44%) in the catchment, with an average slope of approximately 6°. The

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typical soil in the catchment is a readily erodible purple soil with abundant

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macropores, which could contribute over 87% of the flow (Wang et al., 2015). The

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soil thickness on the slopes varies from 25 cm to 60 cm. Mudrock with visible fine

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ACCEPTED MANUSCRIPT fractures lies beneath the shallow purple soil and overlies an impermeable sandstone.

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Other land use types of the catchment include forests (woods) (35%), paddy fields

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(11%) and residential areas (10%). Considering the application of agricultural

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chemicals to farmlands and their potential rapid transport via preferential flow in

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dissolved and colloid-associated forms, the subsurface transport of colloids from a

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sloping farmland plot on the upper slope and the dynamics of colloid concentrations

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in three lowland shallow wells were investigated in this study.

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2.2 Methods

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2.2.1 Monitoring of the sloping farmland plot

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A farmland plot (0.15 hm2) with an average slope of 6° was constructed on the upper

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slope in the catchment (Fig. 1). Details on the experimental plot can be found in

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Zhang et al. (2015). Briefly, rainfall was recorded automatically at 15-min intervals

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by a tipping bucket. A conflux groove was constructed in a trench at the lower end of

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the profile of the plot to collect the fracture flow from the interface of the fractured

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mudrock and the impermeable sandstone. Fracture flow discharge was measured by a

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customized tipping bucket gauge at 15-min intervals. Almost no fracture flow occurs

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during the dry season from early December to the following April. During the rainy

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season from May to November, fracture flow is the dominant flow type (Zhao et al.,

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2016).

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ACCEPTED MANUSCRIPT In this system, fracture flow was sampled daily with a syringe pump at the outlet of

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the conflux groove as the flow occurred at the beginning of the rainy season in 2013.

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In cases of large rainfall events, sampling of the fracture flow was carried out twice

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per day at 08:00 am and 20:00 pm. The whole sampling period lasted for eight months

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from May through December until fracture flow ceased. All the flow samples were

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stored in glass bottles prior to analysis. After the flow sampling period, surface

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sediment (≤5 cm) in a small settling pond (200 cm long, 100 cm wide and 150 cm

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deep) receiving fracture flow was collected manually in January 2014. The sediment

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was dispersed in the fracture flow water and sonicated in a water bath sonicator (KQ-

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3000VDE, Huqin Equipement Co., Ltd., Shanghai, China) for 30 min before settling.

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The suspension with colloids (<2 µm) was recovered by the pipette method according

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to Stokes’ law and was subsequently oven dried (60 °C). In addition, soil and rock

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samples were collected in the plot. The sampling depth was 0-15 cm (plough layer)

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for the soil samples and 5 cm below the interface between the soil layer and the

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weathered mudrock for the rock samples. Three upper locations near the trench were

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sampled. The samples were air-dried and sieved through 150 µm for analysis.

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2.2.2 Monitoring of the well system

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Five wells were constructed and are used as the only sources of drinking water for the

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local residents. In the lower part of the catchment, three of these wells were selected

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ACCEPTED MANUSCRIPT and monitored for water and colloid dynamics (Fig. 1). Two wells (i.e., the

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Zhaoxingqiang well and Zhangfei well) that exhibit different perturbations were

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chosen. The Zhaoxingqiang well is located at the footslope, and the water is extracted

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only via bailing. The Zhangfei well is located in the valley close to the outlet of the

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catchment, and the water is extracted via pump withdrawal. In addition, a third well

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(the Xinjing well) that experiences no human perturbations and that is located in the

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woods on the mid-slope below the sloping farmland plot was also selected as a

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monitoring well. Given the much lower fluctuations of the well water compared with

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the fluctuations of fracture flow in the sloping farmland plot, water was sampled

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biweekly from the three wells. The water level (recorded as the depth below the

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ground) and temperature were measured manually while sampling. Water samples

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from the wells were collected by a low flow rate syringe pump at 30-50 cm below the

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water surface. The sampling period lasted for one year starting in January 2013. After

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the last water sampling, surface sediments (≤5 cm) at the bottoms of the Xinjing and

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Zhaoxingqiang wells were collected through core sampling in January 2014. The

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collected sediment samples were dispersed in the well water, and colloid fractions

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were recovered using the same method that was described for the sloping farmland

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plot. Similarly, soil and rock samples from the farmland and/or forestland at the mid-

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slope and foot slope close to each well (approximately 5 m away) were collected,

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ACCEPTED MANUSCRIPT each with three replicates. The soil sampling depths were 0-15 cm and 35-40 cm for

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the farmland and 0-10 cm for the forest. The sampling depth for the rock layer was 5

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cm below the soil-rock interface. The samples were treated in the same way as the soil

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and rock samples collected from the sloping farmland plot.

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2.2.3 Sample analysis

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Fracture flow and well water samples were analyzed for water chemistry and colloid

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concentrations. pH, electrical conductivity (EC) and colloid concentration were

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determined within 12 h after sampling. pH and EC were measured by a pH and EC

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meter (SensION+MM150, Hach Company, Loveland, CO., USA). The PSD of the

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groundwater samples was measured by a laser scattering PSD analyzer (LA950,

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Horiba, Ltd., Kyoto, Japan); detailed information on this process can be found in

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Zhang et al. (2016). Colloid numbers in the groundwater samples were calculated by

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Eq. 1 (Supporting Information). Colloid absorbency in all the water samples was

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determined for colloid concentration by a spectrophotometer (Tu-1810, Purkinje

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General Instrument Co., Beijing, China) at a wavelength of 400 nm using a calibration

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curve of the colloid mass concentration (mg/L) against absorbance. Additional

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absorbance and PSD measurements were carried out on the water samples after

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ultrasound treatment (2 min, 100 W) in a water bath sonicator. Then, the water

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samples were filtered through 0.45-µm filters and used for the determination of the

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ACCEPTED MANUSCRIPT dissolved organic carbon (DOC) and divalent cation (Ca2+ and Mg2+) concentrations.

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The DOC concentration was measured by a continuous flow analyzer (Auto Analyzer

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3, SEAL Analytical, Norderstedt, Germany). Cation concentrations were determined

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by ion chromatography (ICS-900, Dionex, Sunnyvale, CA, USA).

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The colloids that were separated from the surface sediments collected at the fracture

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flow outlet and well bottoms, as well as the soil and rock samples from the two

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systems, were analyzed for organic matter (OC) content, carbonate content, mineral

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composition and δ13C content. The OC and carbonate contents were measured by the

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potassium dichromate method and titration method, respectively. The mineral

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composition was determined by X-ray diffraction (D/max-2500, Rigaku Corporation,

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Japan). The δ13C content was determined by an isotope ratio mass spectrometer

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(IRMS Delta V Plus, Thermo Scientific, USA) coupled to an elemental analyzer (1112

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Flash EA, Thermo Scientific, USA).

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2.3 Statistical analysis

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Partial redundancy analysis (pRDA), performed by variation partitioning analysis

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(VPA) using “vegan” in R, was conducted to identify the quantitative variation in the

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contribution of flow discharge/well depth and the physicochemical groundwater

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parameters to colloid dynamics. pRDA has already been used in water quality studies

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by other researchers (Buffam et al., 2016; Heier et al., 2010). Moreover, structural

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ACCEPTED MANUSCRIPT equation modeling (SEM), performed by “lavaan” in R (Rosseel, 2012), was

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conducted to further test the relationships between colloid dynamics in the fracture

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flow and the rainfall hydraulics and flow chemistries. Multiple goodness-of-fit criteria,

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including the χ2 test (P>0.05), the goodness-of-fit index (GFI>0.90) and the root mean

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square error of approximation (RMSEA<0.08), were used in the SEM. In addition,

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Pearson correlation and multi-linear regression (stepwise method) analyses were used

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in combination to evaluate the relationship between colloid concentration dynamics

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and flow/water physicochemical properties. Moreover, one-way ANOVA (S-N-K test

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for post hoc multiple comparisons, P<0.05) was applied in the significance difference

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tests for the OC, carbonate and δ13C contents of the environmental samples. The

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software package SPSS 17.0 and Origin 8.0 for Windows were used for these

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statistical analyses.

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3. Results and discussion

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3.1 Dynamics of fracture flow and well water

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In 2013, fracture flow did not occur until the onset of a rain event on 1 May

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(rainfall=17.6 mm, Imax=5.6 mm/h) (Fig. 2). Afterwards, fracture flow lasted

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throughout the course of the rainy season and ceased in December. Generally, distinct

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peaks of fracture flow discharge occurred following the perturbations caused by rain

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events. Specifically, sharp peaks of fracture flow discharge occurred during heavy

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ACCEPTED MANUSCRIPT storms only (e.g., on 18 July, 22 July and 19 September). Descriptive statistics of the

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hydraulic and physicochemical parameters of the fracture flow are presented in Table

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1. The fracture flow discharge showed the largest variation (CV=1.87), between 0.089

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and 67.22 L/min, and the corresponding daily flow depth varied between 0.004 and

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14.64 mm/day (CV=1.31). The well-developed preferential pathways (i.e., soil

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macropores and mudrock fractures) facilitated rapid rainwater infiltration and

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subsurface flow recession, resulting in a substantial variation in flow discharge within

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a short time in response to a rain event. The fracture flow was chemically neutral to

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alkaline. Flow pH showed little variation (CV=0.05) between 6.75 and 8.81, which

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could be ascribed to the buffering capacity of the calcareous purple soil in this region.

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The EC of the fracture flow varied between 441 and 769 µS/cm. The mixing of the

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dilute rainwater (EC<30 µS/cm) with the mobile soil pore water was responsible for

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the decrease in fracture flow EC. Divalent cation (Ca2+ and Mg2+) concentrations in

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the fracture flow showed moderate variations. The average Ca2+ and Mg2+

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concentrations were 83.36 mg/L and 15.70 mg/L, respectively, and were in a similar

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range as those of the fracture flow from the karst limestone aquifers (Atteia et al.,

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1998; Yang et al., 2013). The DOC concentrations of the fracture flow showed greater

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variation (CV=0.46) between 0.28 and 3.82 mg/L. Higher DOC concentrations were

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generally observed in the summer (from June to September), when the intensive dry-

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ACCEPTED MANUSCRIPT wet cycles facilitated the release of soil organic carbon into the soil pore water and

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subsequently appeared in the fracture flow.

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The water levels in the Xinjing and Zhaoxingqiang wells gradually decreased to the

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lowest levels at the end of the dry season (April 2013) (Figs. S1-S2, Supporting

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Information). In contrast, the water level in the Zhangfei well, which is located near

329

the catchment outlet, showed obvious fluctuations (Fig. S3) that could have been

330

caused by the frequent pumping of drinking water. With the arrival of the rainy season

331

in early May, the water levels of the three wells gradually increased and exhibited the

332

first peaks in late June, while other distinct peaks also occurred after heavy storms. In

333

comparison to the distinct peaks of fracture flow discharge observed in the sloping

334

farmland plot, the three wells exhibited fewer water level peaks but with much longer

335

durations, indicating the slower and lagged response of the shallow well water to rain

336

events.

337

The water temperature of the Xinjing and Zhaoxingqiang wells varied between 10.1

338

and 25.1 °C (Table S1), with an obvious peak occurring in August (Figs. S1-S2). The

339

water temperature of the Zhangfei well remained at a plateau of approximately 19 °C

340

from early August to early November (Fig. S3), which is much lower than those of the

341

other two wells. The smallest variation in water temperature in this well (CV=0.07,

342

Table S1) was due to its largest depth below ground (>4 m), as compared to the depths

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ACCEPTED MANUSCRIPT of the other two wells, so that the deeper groundwater was less affected by air

344

temperature. The pH of the well water was in the same range as that of the fracture

345

flow, but the EC of the well water was significantly higher (P<0.01) than that of the

346

fracture flow. Both the EC and the major divalent cation concentrations (Ca2+ and

347

Mg2+) of water from the Zhangfei well were significantly higher than those from the

348

other two wells (P<0.01), probably due to the release of cations from thicker soil and

349

rock layers into the groundwater. The DOC concentration of the Xinjing well water

350

increased rapidly in early May and reached a maximum (11.8 mg/L) in mid-June (Fig.

351

S1). The increase in the DOC concentration was mainly ascribed to the early leaching

352

of the organic carbon pool in the shallow soil, which was supplied by decaying wood

353

litter that had accumulated in the dry season. A lower average DOC concentration was

354

observed in the Zhaoxingqiang well (Fig. S2), and the lowest level was found in the

355

Zhangfei well (Fig. S3), indicating the retention of DOC by the soil during its

356

downward transport.

357

3.2 Dynamics of colloids in shallow groundwater

358

The daily colloid concentration of the fracture flow varied from 0.54 to 31.90 mg/L,

359

with an average concentration of 1.64 mg/L (Table 1). A high colloid concentration

360

lasted approximately 1 week, with an average concentration of 5.59 mg/L observed

361

after the commencement of the fracture flow on 1 May (Fig. 2). This phenomenon

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ACCEPTED MANUSCRIPT was unique and specific to the very beginning of the rainy season following a long dry

363

period, during which the differential capillary stresses might lead to soil matrix and/or

364

soil pores weakening and further colloid generation (Michel et al., 2010). The

365

abundant mobilized colloids were continuously transported to the fracture flow due to

366

the advancement of the wetting front of the newly infiltrated rainwater, which was

367

responsible for the clustering of high colloid concentration. This finding is consistent

368

with other findings from column studies (Majdalini et al., 2008; Mohanty et al.,

369

2015b). Afterwards, the colloid concentration generally stayed at a lower base level

370

(1.15 mg/L) during the non-rainy days, except for a few peaks that were several or

371

even tens of times the base level in response to heavy storms. The highest colloid

372

concentration was 31.90 mg/L and was observed during the storm on 19 September

373

(rainfall=92.2 mm, Fig. 2). The second highest colloid concentration was 27.50 mg/L

374

and was observed during the storm on 22 July (rainfall=120.1 mm, Fig. 2). The

375

durations of elevated colloid levels (less than one day) were shorter than those of

376

elevated flow discharges (several days), which is consistent with the reported faster

377

increases in colloid concentration compared to those of the subsurface flow discharge

378

during rain events in a number of field studies (El-farhan et al., 2000; McKay et al.,

379

2000; Schiperski et al., 2016; Toran et al., 2006; Zhang et al., 2016).

380

In the three lowland wells, the colloid mass concentration varied between 0.20 and

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ACCEPTED MANUSCRIPT 11.66 mg/L, and the corresponding number concentration varied between 1.62×107

382

and 1.07×109 colloids/L (Table 2). The colloid populations observed in this study were

383

1-3 orders of magnitude lower than those in the granitic groundwater from the

384

Grimsel Test Site (Degueldre et al., 1989) and in coastal groundwater (Rani and

385

Sasidhar, 2011). Colloid concentrations of the groundwater in the wells varied in a

386

much smaller range and exhibited much lower peak concentrations and less temporal

387

variations than those of the fracture flow from the sloping farmland. In addition,

388

decreasing trend of the average colloid concentration from 2.72 mg/L in the Xinjing

389

well to 1.42 mg/L in the Zhangfei well close to the catchment outlet was observed,

390

which could be attributed partially to the increased retention of subsurface colloid

391

transported along the streamline. A similar colloid concentration decline with distance

392

in the down-gradient wells was previously reported (McKay et al., 2000). However,

393

the temporal dynamics of the colloid concentration showed different patterns among

394

the three wells. In the Xinjing well which was not subjected to human activity, the

395

colloid mass concentration increased gradually from less than 2.00 mg/L at the end of

396

the dry season to a peak of 7.62 mg/L in mid-June, followed by a rapid decrease to

397

below 2.00 mg/L (Fig. S1). This observation is similar to observations from the

398

sloping farmland plot in which colloid concentration increased to a plateau in

399

response to rain events at the early rainy season. However, this is not the case for the

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ACCEPTED MANUSCRIPT other two wells. In the Zhaoxingqiang well, a distinct colloid concentration peak

401

(11.66 mg/L) occurred on 2 July in response to the previous consecutive storms (30

402

June to 1 July, cumulative rainfall of 203.2 mm) (Fig. S2). A much smaller response

403

of the colloid concentration to these storms was observed in the Zhangfei well (Fig.

404

S3). However, a secondary colloid concentration peak in the Zhaoxingqiang well and

405

major peaks in the Zhangfei well appeared during the dry season (by the end of April),

406

when water levels were very low and continuously declined in both wells. These

407

peaks were mainly the result of perturbations related to water extraction. The frequent

408

bailing from the Zhaoxingqiang well and pump withdrawal of water from the

409

Zhangfei well at low water levels facilitated the re-suspension of previously deposited

410

colloids at the well bottoms. Similarly, Rani and Sasidhar (2011) reported that the

411

physical perturbations of water extraction led to the artifact of increased colloid

412

concentrations in the wells. In addition, sediment re-suspension has been reported to

413

contribute up to 55% of the natural colloids suspended in the water column in a lake

414

(Xu et al., 2018).

415

The effects of the ultrasound treatment of the water bath on the colloid concentration

416

of the groundwater samples are presented in Figs. S4-S5. The Cus/C ratios were all

417

higher than 1:1 (the dotted line), regardless of the fracture flow samples or well water

418

samples, indicating that the colloids in the shallow groundwater existed as aggregates.

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ACCEPTED MANUSCRIPT In the present study, the high electrical conductivities and particularly high Ca2+

420

concentrations (57.04-107.03 mg/L for the fracture flow and 77.00-207.03 mg/L for

421

the well water) that far exceeded the critical coagulation concentration (CCC) of

422

purple soil (24 mg/L) and mudrock (16 mg/L) particles (Zhang et al., 2016) would

423

have extensively neutralized the negative charges in the diffuse electric double layer

424

(EDL) of the colloids. The resulting compression of the EDL further promoted the

425

colloid coagulation. Similar findings of divalent cation-induced colloid coagulation

426

were reported by other studies (Kuno et al., 2002; Shiyan et al., 2014; Xu et al., 2018).

427

The PSD of fracture flow samples from the hillslope has been explored in our

428

previous single-event based studies (Zhang et al., 2015, 2016). Suspended particles in

429

the fracture flow ranging in size from submicron to more than 100 µm were observed

430

while the colloid (<2 µm) presence was lower than 1%. However, the PSD of the well

431

water exhibited a different pattern from that of the fracture flow, as shown in Fig. 3. A

432

smaller size range from submicron up to 40 µm, a slightly higher colloid percentage

433

(2.4-7.0%) and smaller median sizes (4.2-10.0 µm) were observed (Table 3). Low

434

percentages of colloid in the fracture flow and lowland wells were ascribed to colloid

435

diffusion into dead-end voids, such as the soil matrix or fracture apertures, during the

436

subsurface transport process. This matched the observations from other studies, e.g.,

437

Alonso et al. (2007), Mondal and Sleep (2012), Zvikelsky and Weisbrod (2006). The

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ACCEPTED MANUSCRIPT much smaller maximum size of suspended particles and the higher cumulative particle

439

percentage from colloids to coarse particles (2-10 µm) in the well water relative to the

440

fracture flow implies a greater retardation of large particles (>10 µm) during their

441

transport in the catchment. Gravity sedimentation acts as the main cause of

442

immobilization of large particles, especially in horizontal flow conduits and/or when

443

the flow rate decreases. However, the gravity effect of colloids exhibited an

444

insignificant influence on colloid mobilization and transport in finely fractured aquifer

445

in the present study, considering the intense driving force of macropore flow and/or

446

fracture flow that enabled the transport and enrichment of coarse particles in the well

447

water. Monhanty et al. (2015a) also reported the irrelevance of gravity sedimentation

448

on colloid transport, as the 1.8-µm colloids recovered to a greater extent than the 0.5-

449

µm colloids during transport in a macroporous soil. The environmental implications

450

of the PSD results for regions characterized by finely fractured aquifers are that

451

particle investigation of the groundwater in these areas should include but not be

452

limited to colloids; coarse particles up to 10 µm should also be taken into account.

453

Apparent shifts towards smaller groundwater PSDs following ultrasound treatment

454

are also shown in Fig. 3. The colloid percentage increased by a factor of 3.0, 2.5 and

455

13.8 in the Xinjing well, Zhaoxingqiang well and Zhangfei well, respectively.

456

Breakage of the coagulated colloids, especially in the high-salinity Zhangfei well, led

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24

ACCEPTED MANUSCRIPT to the increase in the colloid presence following the ultrasound treatment.

458

3.3 Physicochemical perturbations of colloid dynamics in the fracture flow and

459

shallow groundwater

460

Due to the large temporal variation in the fracture flow discharge (CV=1.87, Table 1),

461

it is reasonable to separate the flow discharge into high and low ranges according to

462

the median flow discharge (1.3 L/min) for further detailed analyses via pRDA,

463

Pearson correlation and multi-linear regression analysis.

464

According to the pRDA, a total of 67.84% and 84.28% of the variations in colloid

465

dynamics in the fracture flow could be explained for the general cases and the high

466

flow discharge cases, respectively, to which flow discharge contributed more than 45%

467

(Fig. 4). This finding indicates the dominance of flow discharge, instead of flow

468

chemistry, on colloid mobilization and transport in the fracture flow from the hillslope.

469

SEM was employed to explore the routes and extensions of the physicochemical

470

factors on colloid dynamics in the fracture flow. Our model explained 60% of the

471

variance in colloid dynamics in the fracture flow (Fig. 5). Flow discharge can impact

472

colloid dynamics directly (λ=0.70, P<0.001) and can also impose an indirect effect on

473

colloid dynamics by affecting flow chemistry (λ=-0.25, P<0.01). In contrast, flow

474

chemistry showed a half less importance of direct impact (λ=0.31, P<0.001) on

475

colloid dynamics. This is in accordance with pRDA in which flow discharge acted as

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ACCEPTED MANUSCRIPT the primary and positive factor influencing colloid dynamics.

477

In addition, the colloid concentration was significantly positively correlated with the

478

fracture flow discharge, with a Pearson correlation coefficient of 0.756, which is

479

much greater than that of the chemical parameters (Table 4). Similarly, flow discharge

480

showed the largest influence (53.27%) on colloid mobilization and transport dynamics

481

among all the physicochemical parameters according to the multi-linear regression

482

analysis (Table 4). For the fracture flow samples taken at high discharges (>1.3

483

L/min), an increased correlation (0.885) of colloid dynamics with flow discharge and

484

a slightly larger effect (58.09%) of flow discharge on colloid transport was observed.

485

This result agrees with the results of the pRDA and SEM analyses. The fracture flow-

486

derived shear force, which corresponds to flow discharge and possibly coupled with

487

the air-water interface especially following the dry period, is the main mechanism for

488

colloid scoring, mobilization and transport in the fracture flow. Hydraulic shearing

489

has also been reported as the dominant force controlling colloid dynamics in fractured

490

geologic media elsewhere in response to rainfall (Mohanty et al., 2015a; Shevenell

491

and McCarthy, 2002; Weisbrod et al., 2002).

492

In contrast, for the low flow discharge cases, only 20.73% of the variation in colloid

493

dynamics in the fracture flow was explained and was due nearly exclusively to flow

494

chemistry (Fig. 4). The colloid concentration showed insignificant correlations with

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26

ACCEPTED MANUSCRIPT flow discharge in this situation (Table 4). The influence of the fracture flow discharge

496

decreased by half more (from >50% to 21.67%, Table 4) and became the secondary

497

factor for colloid mobilization and transport, unlike in the two aforementioned

498

situations. Therefore, the physical perturbation of hydraulic infiltration exhibits a

499

primary effect on colloid mobilization and transport dynamics in the fracture flow

500

from the hillslope in general. In particular, this effect retreats to secondary importance

501

behind flow chemistry (pH, Mg2+ and DOC) when the fracture flow discharge is low

502

(<1.3 L/min). Increased hydraulic retention of fracture flow occurs at low flow

503

discharges, facilitating the interactions of colloids with groundwater. The pH (7.21-

504

8.81, 8.10 on average) of fracture flow at low discharges was significantly higher

505

(P<0.05) than that of the general fracture flow (6.75-8.81, 7.96 on average) and flow

506

at high discharges (6.75-8.74, 7.82 on average). According to the calculations from

507

Zhou et al. (2011), the surface potential (ζ-potential) of soil colloids increased from -

508

49 mV to -68 mV (more negative) when the solution pH increased from 7.0 to 9.0.

509

Therefore, the significantly increased flow pH at low discharges leads to the increase

510

in electrostatic repulsion between colloids and the solid surfaces and further favors

511

colloid mobilization into fracture flow.

512

The depth variations in the well water, which were directly affected by subsurface

513

flow discharge, offered almost no explanations for colloid dynamics in the lowland

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27

ACCEPTED MANUSCRIPT wells (Fig. 4). Water chemistry accounted for 21.73-70.11% of the explained

515

variations (24.35-77.03%), indicating the dominant role of water chemistry on colloid

516

dynamics in the lowland wells. With respect to water chemistry, EC and Mg2+ in most

517

cases, as well as Ca2+ in certain cases (Tables 4-5), showed a significant negative

518

effect on colloid dynamics in both the well water and fracture flow. This phenomenon

519

agrees with the DLVO theory. With the remarkable decrease in EC or divalent cation

520

concentrations in groundwater, for example due to the incorporation of dilute

521

rainwater, irrigation and/or recharge water, an increased repulsive energy barrier could

522

develop between the dispersed colloids and the solid surfaces (soil matrix and/or

523

pore/fracture surfaces) and thus favor the dispersion and mobilization of colloids

524

(Ryan and Gschwend, 1994; Zhang et al., 2012). Many researchers have reported a

525

negative correlation between the stability of colloids and solution ionic strength in

526

column-scale studies (Albarran et al., 2014; Masciopinto and Visino, 2017; Morales et

527

al., 2011; Zhang et al., 2017) and in stream water in a small watershed (Mills et al.,

528

2017).

529

In contrast, the DOC exhibits a general positive correlation with groundwater colloid

530

concentration and a dominant effect on colloid dynamics in the Xinjing well. The

531

sorption of DOC molecules to natural colloids has been widely recognized, leading to

532

the modified surface negative charges of colloids in most cases (Xu et al., 2018). This

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ACCEPTED MANUSCRIPT modification would promote the electric repulsions between colloids and solid

534

surfaces and further increase the chances of colloid mobilization. Other researchers

535

have reported the promotion of colloid stability by dissolved organic matter in natural

536

groundwater as well as improved colloid transport in the presence of DOC (Cheng

537

and Sailers, 2015; Morales et al., 2011; Shiyan et al., 2014). Moreover, the adsorbed

538

DOC could also block the deposition sites, especially at a high flow rate (Yang et al.,

539

2015), leading to the enhanced mobility of colloids in groundwater. However, no

540

significant correlations between the colloid concentration and water chemistries were

541

found for the Zhangfei well. Instead, there was a distinct negative correlation between

542

the water temperature and the colloid concentration in this well (Table 5), a trend that

543

was not observed in the fracture flow or the other two wells. This observation implies

544

that, provided the highest EC and cation concentrations which are favorable for

545

colloid coagulation in this well, the small increases in water temperature (3.3°C) from

546

May to October could lead to the enhanced Brownian motion of the coagulated

547

colloids. The improved collision of the inter-coagulated colloids potentially promotes

548

the formation of large particles that could easily settle to the well bottom due to

549

gravity sedimentation, resulting in fewer colloids present in the well water. This

550

hypothesis is in agreement with a laboratory study in which a decrease of the stability

551

of the latex colloidal system with increasing of solution temperature was observed

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ACCEPTED MANUSCRIPT (Garcia-Garcia et al., 2006). This relationship does not exist for the other two wells or

553

the fracture flow, attributable to the much lower salinities in these cases.

554

3.4 Groundwater colloid characteristics and source identification

555

Mineral composition and calcium carbonate, organic matter (OC) and δ13C abundance

556

were used to characterize groundwater colloids and soil and rock samples from upper

557

geological media for the identification of colloid sources.

558

A number of minerals (quartz, plagioclase, feldspar, calcite, hematite, montmorillonite,

559

illite, chlorite, etc.) were detected in all three categories of solids but in different

560

proportions (Fig. S6). Quartz and illite were the two major minerals in the soil and

561

underlying mudrock in the sloping farmland plot, while quartz was the dominant

562

mineral in both the soil and mudrock samples collected from the farmlands and

563

forestlands near the wells. At both the sloping farmland site and the well sites, the

564

lowest content of quartz and the highest content of illite were observed in the

565

groundwater colloids and not the soil or mudrock samples. This observation is

566

consistent with the findings of Lei et al. (2004), who also found a higher proportion of

567

illite in the mineral colloids than in the purple soil itself. This is also the case for the

568

colloids in the subsurface water from the adit of an abandoned mine (Filella et al.,

569

2009). In addition, an increase in the percentage of illite in colloids in a river was

570

ascribed to the high load of soil colloids in the watershed during the snow melting

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ACCEPTED MANUSCRIPT period (Chanudet and Filella, 2008).

572

As presented in Fig. S7, groundwater colloids in both the fracture flow and well water

573

had a significantly (P<0.05) higher calcium carbonate content (>25%) than the soils

574

or mudrocks of the nearby geological media. The enriched carbonate content in the

575

groundwater colloids could be explained by the precipitation of calcium carbonates

576

facilitating the physical inter- and intra-cementing of colloids during the process of

577

colloid dispersion and generation (Zhao et al., 2012). In addition, the hematite could

578

also act as the cementing material. As shown in the inset graphs in Fig. S6,

579

groundwater colloids had the highest proportion of hematite, whose presence can

580

facilitate the chemical bonding of organic substances to mineral particles (Schulten

581

and Leinweber, 2000). Therefore, groundwater colloid complexation by dissolved

582

organic matter potentially increases the OC content of colloids.

583

In the sloping farmland plot, the OC content of the colloids (0.91%±0.06%) in the

584

fracture flow showed an insignificant difference (P>0.05) to that of the soil

585

(0.71%±0.13%) while both of them were significantly higher (P<0.05) than that of the

586

rock (0.2%±0.02%) (Fig. S7). Groundwater colloids from both the Xinjing well and

587

Zhaoxingqiang well had significantly higher (P<0.05) OC contents (1.02%±0.11%

588

and 0.96%±0.08%, respectively) than the rocks (<0.21%) of the upper farmlands

589

or forestlands (Fig. S7). These findings might exclude the possibility that fractured

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31

ACCEPTED MANUSCRIPT rocks in the hillslope or the upper farmlands and forestlands of the lowland wells are

591

the source of the groundwater colloids. In addition, the OC content of the

592

groundwater colloids was slightly higher than that of the shallow soil of the upper

593

farmlands. Considering the increased OC content of colloids due to the complexation

594

of dissolved organic matter in the groundwater, shallow soil from the upper farmland,

595

rather than the forest soil, may act as the major source of groundwater colloids in the

596

present study.

597

In addition to the above parameters, δ13C was also used to help identify the source of

598

the groundwater colloids. In the sloping farmland plot, the δ13C abundance of the

599

colloids (-15.13‰±0.24‰) in the fracture flow was not significantly different

600

(P>0.05) from that of the soil (-14.45‰±0.46‰), but both of them were significantly

601

lower (P<0.05) than that of the underlying mudrock (-9.24‰±0.27‰) (Fig. 6). This

602

result indicates that the colloids in the fracture flow originate from the shallow soil of

603

the sloping farmland, which agrees with the results of the source identification based

604

on the mineral composition, organic matter and calcium carbonate. In the Xinjing well,

605

the δ13C abundance of colloids (-14.61‰±0.18‰) was slightly lower (P>0.05) than

606

that of the shallow soil (-14.34‰±0.33‰) from the upper farmland. These results

607

were markedly different from those of the other geological samples. In the

608

Zhaoxingqiang well, groundwater colloids showed a similar δ13C abundance (-

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32

ACCEPTED MANUSCRIPT 14.61‰±0.18‰) as that of the shallow soil (-15.27‰±0.07) and underlying mudrock

610

(-14.90‰±0.43‰) of the upper farmland, while all of these samples had markedly

611

lower (P<0.05) δ13C abundances than those of the deep farmland soil and forest soil

612

and rocks. This phenomenon implies little contributions from nearby forestlands to

613

the groundwater colloids in the well. As mudrock has been demonstrated to be an

614

unlikely source of the groundwater colloids, the δ13C abundances also suggest that

615

shallow farmland soil is the major source of colloids present in the lowland wells.

616

This is the first attempt to identify groundwater colloid origination by using δ13C.

617

However, the incorporation of δ13C in colloid source tracing should be combined with

618

other characteristic parameters, e.g., mineral composition and calcium carbonate and

619

organic matter content, for better reliability.

620

4. Conclusion

621

Our study provides quantitative evidence indicating that different mechanisms

622

dominate colloid dynamics in different groundwater patterns. Hydraulic perturbations

623

of rainwater infiltration exhibit a large and direct effect on colloid mobilization and

624

transport via the fracture flow from the upper hillslope. Subsequently, the variations in

625

physicochemical properties of flow water along subsurface transport pathways exert a

626

major effect on colloid dynamics in the lowland wells. These understandings of

627

colloid mobilization and transport mechanisms are crucial to prioritizing the

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ACCEPTED MANUSCRIPT monitoring of flow/water parameters, which may govern colloid dynamics in shallow

629

groundwater. In addition to the high concentrations of colloids (maximums of 31.90

630

mg/L and 11.66 mg/L in the facture flow and well water, respectively), which exist in

631

coagulated forms, a high percentage (47.7-92.0%) of coarse particles (2-10 µm)

632

indicate the importance of including coarse particles in the monitoring scheme of

633

shallow groundwater, especially in finely fractured aquifers and/or karst aquifers.

634

Moreover, the new approach proposed in this study for source identification of

635

groundwater colloids by simultaneous analyses of δ13C and mineral composition,

636

organic matter and carbonate content in groundwater colloids and the upper

637

geological samples would be useful for colloid-facilitated contaminant transport

638

studies in large catchments with spatially varying geological media.

639

Acknowledgements

640

This study was supported by the National Natural Science Foundation of China (Grant

641

Nos. 41790431 and 41601539), the 135 Strategic Program of the Institute of

642

Mountain Hazards and Environment (Grant No. SDS-135-1702) and the Key

643

Research Program of Frontier Sciences, Chinese Academy of Sciences (QYZDJ-

644

SSW-DQC006).

645

References:

646

Albarran, N., Degueldre, C., Missana, T., Alonso, U., García-Gutiérrez, M., López, T., 2014. Size

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ACCEPTED MANUSCRIPT 647

distribution analysis of colloid generated from compacted bentonite in low ionic strength

648

aqueous solutions. Appl. Clay Sci. 95, 284-293.

651 652 653 654 655 656 657 658

rock of a deep geological repository. Phys. Chem. Earth 32, 469-476. Atteia, O., Perret, D., Adatte, T., Kozel, R., Rossi, P., 1998. Characterization of natural colloids from a river and spring in a karstic basin. Environ. Geol. 34, 257-269. Buffam, I., Mitchell, M.E., Durtsche, R.D., 2016. Environmental drivers of seasonal variation in green roof runoff water quality. Ecol. Eng. 91, 506-514.

RI PT

650

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ACCEPTED MANUSCRIPT Figure captions Fig. 1. Distributions of the farmland plot and three lowland shallow wells (well 1#: Zhaoxingqiang well; well 2#: Zhangfei well; well 3#: Xinjing well) investigated in the Jieliu catchment.

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Fig. 2. Dynamics of the discharge, colloid, cation and DOC concentrations, pH and EC of the fracture flow in the 0.15 hm2 sloping farmland plot in 2013. Fig. 3. Particle size distributions of well water samples from the three lowland wells following storm events from 30 June to 1 July. US treatment denotes the ultrasound treatment (2 min, 100 W) of the fracture flow and well water samples in a water bath sonicator.

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Fig. 4. pRDA results of groundwater colloid dynamics explained by discharge/water depth and chemical factors.

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Fig. 5. Structural equation model showing the direct and indirect effects of flow discharge and flow chemistries on colloid dynamics in fracture flow. The solid and dashed arrows indicate significant and insignificant relationships, respectively. R2 denotes the proportion of explained variance of a certain variable by the model. Numbers adjacent to the arrows are path coefficients with the significance levels of *P<0.05, **P<0.01, ***P<0.001. A satisfactory fit of our data by the model is reflected by χ2=0.321, df=1, P=0.571, GFI=0.99 and RMSEA=0.00.

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Fig. 6. Comparisons of δ13C abundances between groundwater colloids (recovered from the fracture flow or well water) and the nearby soil and rock samples of upper locations in the farmlands and forestlands.

ACCEPTED MANUSCRIPT Table 1 Descriptive statistics of the hydraulic and physiochemical parameters of the fracture flow and colloid concentration in the sloping farmland plot

pH

EC (µS/cm)

Ca2+ (mg/L)

Mg2+ (mg/L)

DOC (mg/L)

Colloid (mg/L)

67.22 0.089 3.32 6.21 1.87

14.64 0.004 2.06 2.69 1.31

8.81 6.75 7.96 0.41 0.05

769 441 654 60.84 0.09

107.03 57.04 83.36 9.61 0.12

24.04 3.11 15.70 3.23 0.21

3.82 0.28 1.41 0.64 0.45

31.90 0.54 1.64 2.85 1.74

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SD: standard deviation, CV: coefficient of variation

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Flow depth (mm/day)

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Max Min Mean SD CV

Discharge (L/min)

ACCEPTED MANUSCRIPT Table 2 Descriptive statistics of colloid mass and number concentrations of the well water

Nmax Nmin Mmax Mmin Mean SD CV

8

7.00×10 8.82×107 7.62 0.96 2.72 1.77 0.65

Zhaoxingqiang well

Zhangfei well

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1.07×10 4.74×107 11.66 0.52 2.21 2.24 1.01

5.06×108 1.62×107 5.51 0.24 1.42 1.23 0.87

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Xinjing well

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maximum and minimum mass concentration (mg/L)

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Nmax and Nmin: maximum and minimum number concentration (colloids/L); Mmax and Mmin:

ACCEPTED MANUSCRIPT Table 3 Percentages of colloids (<2 µm) and coarse particles (2-10 µm) and median diameter (d50) of suspended particles in lowland wells

<2 µm

<10 µm

d50a (µm)

XJ ZXQ ZF

5.2 (20.9) b 7.0 (24.7) 2.4 (35.4)

97.2 (99.8) 89.2 (98.8) 50.1 (99.2)

4.2 (3.0) 5.3 (3.2) 10.0 (2.5)

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Lowland well

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XJ, ZXQ and ZF denotes Xinjing well, Zhaoxingqiang well and Zhangfei well, respectively.

a: Median particle diameters denotes the particle size at which the cumulative percentage of

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particles reaches 50%.

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b: Numbers in the bracket indicate the situations after ultrasound treatment.

ACCEPTED MANUSCRIPT Table 4 Results of Pearson correlation and linear regression (stepwise method) analyses of colloid concentration with the physicochemical parameters of the fracture flow EC (µS/cm)

Ca2+ (mg/L)

Mg2+ (mg/L)

DOC (mg/L)

Discharge
0.756** 0.798 53.27 0.885** 0.926 58.09 0.097 0.226 21.67

0.087 0.179 11.95 0.052 0.113 7.09 0.280* 0.338 32.41

-0.339** -0.204 13.62 -0.553** -0.101 6.34 -0.087 --d --

-0.135* -0.111 7.41 -0.167 -0.142 8.91 -0.075 ---

-0.319** 0.110 7.34 -0.368** 0.214 13.43 -0.376** -0.288 27.61

0.210** 0.096 6.41 0.247** 0.098 6.15 0.100 0.191 18.31

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Discharge>d50 (n=117)

Ra Rscb NRsc (%)c R Rsc NRsc (%) R Rsc NRsc (%)

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All cases (n=238)

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pH

Discharge (L/min)

d50 denotes the median (50% percentile) discharge.

a: R represents Pearson correlation coefficient of colloid concentration with each parameter. b and c: Rsc and NRsc represent the standardized coefficients (Beta) from the multi-linear

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equation.

*Correlation is significant at the 0.05 level (2-tailed).

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**Correlation is significant at the 0.01 level (2-tailed).

ACCEPTED MANUSCRIPT Table 5 Results of Pearson correlation and linear regression (stepwise method) analyses of colloid concentration with the physicochemical parameters of the well water

Ca2+ (mg/L)

Mg2+ (mg/L)

DOC (mg/L)

0.229 --

-0.522** --

-0.489** -0.285

-0.381* --

0.812** 0.720

--

--

--

--

28.36

--

71.64

0.263 --

0.139 --

0.066 --

-0.668** -0.667

-0.290 --

-0.146 --

0.435* --

Zhaoxingqiang well (n=27)

--

--

--

100

--

--

--

-0.247 --

-0.555** -0.555

-0.308 --

-0.216 --

0.201 --

-0.232 --

-0.290 --

Zhangfei well (n=27)

--

100

--

--

--

--

--

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Xinjing well (n=27)

R Rscb NRsc (%)c R Rsc NRsc (%) R Rsc NRsc (%)

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a

Tem

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EC (µS/cm)

-0.337 --d

(°C) 0.201 --

pH

WL (cm)

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a: R represents Pearson correlation coefficient of colloid concentration with each parameter. b and c: Rsc and NRsc represent the standardized coefficients (Beta) from the multi-linear regression analysis and the normalized values of each Rsc, respectively.

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equation.

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d: -- indicates the corresponding variable is excluded in the standardized linear regression

*Correlation is significant at the 0.05 level (2-tailed). **Correlation is significant at the 0.01 level (2-tailed).

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Highlights Fracture flow and shallow groundwater were monitored

simultaneously.

Hydraulic perturbation generally dominates colloid dynamics in fracture flow.

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Water chemistries dominate colloid dynamics in shallow wells.

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Multi-parameters were jointly used for groundwater colloid source identification.

ACCEPTED MANUSCRIPT Dear Editor,

The authors declare that we have no commercial or associative interest that represents a conflict of interest in connection with this work submitted (i.e., Dynamics and sources of colloids in shallow groundwater in lowland wells and fracture flow in sloping farmland

Best regards, Jun-Fang Cui

Department of Soil and Environment

Chinese Academy of Sciences No. 9, Block 4, Renminnanlu Road Chengdu610041, China

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Institute of Mountain Hazards and Environment

Tel.: +86 28 85213556; fax: +86 28 85222258.

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E-mail address: [email protected]

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by Wei Zhang, Jian-Hua Cheng, Qing-Song Xian, Jun-Fang Cui*, Xiang-Yu Tang, Gen-Xu Wang).