Throughfall Dissolved Organic Matter as a Terrestrial Disinfection

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Throughfall Dissolved Organic Matter as a Terrestrial Disinfection Byproduct Precursor HUAN CHEN, Kuo-Pei Tsai, Qiong Su, Alex T. Chow, and Jun-Jian Wang ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.9b00088 • Publication Date (Web): 29 May 2019 Downloaded from http://pubs.acs.org on May 31, 2019

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ACS Earth and Space Chemistry

Throughfall Dissolved Organic Matter as a Terrestrial Disinfection Byproduct Precursor

Huan Chen,1 Kuo-Pei Tsai,1 Qiong Su,2 Alex T. Chow,1, 3 Jun-Jian Wang4, 5, *

1 Biogeochemistry & Environmental Quality Research Group, Clemson University, South Carolina

29442, United States 2

Water Management & Hydrological Science, Texas A&M University, College Station, Texas

77843, United States; 3

Department of Environmental Engineering and Earth Science, Clemson University, SC 29634,

United States; 4

Guangdong Provincial Key Laboratory of Soil and Groundwater Pollution Control, School of

Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, China; 5

State Environmental Protection Key Laboratory of Integrated Surface Water-Groundwater

Pollution Control, School of Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, China;

* Corresponding Author: Jun-Jian Wang (Mailing Address: 1088 Xueyuan Road, Xili, Nanshan, Shenzhen, Guangdong 518055, China; Email: [email protected])

1 ACS Paragon Plus Environment

ACS Earth and Space Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT More than half of the drinking water supply in the United States originates from forest watersheds, where terrestrial dissolved organic matter (DOM) is known as an important disinfection byproduct (DBP) precursor. Throughfall-derived DOM, a significant contributor of terrestrial DOM, has seldom been evaluated for its formation potential of DBPs. Here, we collected throughfall and leaf extracts of an evergreen (loblolly pine, Pinus taeda L.) and a deciduous tree species (turkey oak, Quercus cerris L.) to explore their seasonal DOM quantity, optical properties, and DBP formation potential. Elevated dissolved organic carbon (DOC) from rainwater (1.2±0.4 mg/L) to pine (26.0±19.7 mg/L) and oak throughfall (38.8±37.8 mg/L) indicated canopies can be a significant DOM source. DOM aromaticity (indicated by specific ultraviolet absorbance at 254 nm) was higher in oak than pine throughfall and higher in throughfall than leaf extracts. The throughfall DOM characteristics were seasonally more stable for the evergreen pine than for the deciduous oak. The specific DBP formation potential of pine and oak throughfall both varied greatly across seasons, with values of 52.7±17.3 and 58.6±15.1 µg/g-DOC for trihalomethanes, 0.82±0.35 and 0.64±0.11 µg/g-DOC for haloacetonitriles, 0.59±0.60 and 0.22±0.05 µg/g-DOC for haloketones, 4.51±2.25 and 4.20±2.76 µg/g-DOC for chloral hydrate, respectively. Estimation of runoff DOC yields from canopies suggests that the highest DOC yield would occur in the fall among all seasons. Results suggest that throughfall DOM is an important and overlooked terrestrial DBP precursors, and its seasonal variation is forest-type-dependent.

Keywords: Throughfall DOM, optical characteristics, disinfection by-products, seasonal variations, pine, oak.


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1. INTRODUCTION Dissolved organic matter (DOM) in source water can react with chemical disinfectants to form carcinogenic disinfection byproducts (DBPs), such as trihalomethanes (THMs) and haloacetonitriles (HANs).1-4 The reactivity of DOM in forming DBPs largely depends on its chemical composition,5,6 which is related to DOM sources.7-9 Terrestrial DOM from the forest floor is widely considered as an important DBP precursor,10-12 particularly during and after storm events.13-15 Forested watersheds can usually provide water with higher quality, which can be used as drinking water after less expensive treatments.16-18 Therefore, they contribute more than half of the drinking water supply in the contiguous United States.16,19 Within the forested watershed, treederived DOM (tree-DOM) has recently been unveiled as one of the critical terrestrial DOM sources.20-22 Trees are estimated to yield comparable amounts of DOM per unit area of landscapes as the streams draining those landscapes, but the chemical nature of tree-DOM is less understood than that of soil DOM or stream DOM.23 Therefore, understanding of the role of tree-DOM, a subset of DOM in forested watersheds, as a possible DBP precursor, will help reduce DBPs by implementing effective and targeted management strategies.24 In forested catchments, when first intercepted by trees, precipitation or rainwater makes its way to the forest floor by two hydrological flow paths: throughfall (rainwater that drips after passing through the canopy) and stemflow (rainwater funnelled to the stem by the canopy).20,21 Most DOM reaches the forest floor as throughfall (40–200 kg C/ha/yr) instead of stemflow (0.2– 50 kg C/ha/yr).20,25 The characteristics of DOM in rainwater are altered when rainwater drains from the canopy as throughfall.20,21,23,26 Throughfall DOM, mainly composed of an autochthonous fraction from trees and an allochthonous fraction from atmospheric deposition,21 can be enriched with aromatic, humic-like, and protein-like compounds.20,21 Aromatic compounds, such as lignin



phenols and aromatic amino acids, are considered to be highly reactive THM precursors.27,28 After reaching the surface of the forest floor/soil, rainwater will travel downhill as surface runoff or will percolate into the groundwater,29 depending on land use and soil cover.30 In southeastern US such as South Carolina, the groundwater is often shallow, which favors the surface runoff over soil infiltration during rainfall events.31 Consequently, the throughfall DOM originating from canopies via the hydrological flow path of throughfall is likely a significant DBP precursor in forested watersheds in this region. However, it is still poorly understood how much throughfall DOM could be exported from the watersheds in South Carolina, and how reactive the throughfall DOM could be to form different types of DBPs. Leaves in the canopy are usually considered as an important source of throughfall DOM and often have large seasonal variations in their biochemistry. For example, Salminen et al.32 found that hydrolyzable tannins, a dominant phenolic content in oak (Quercus robur L.) leaves, decreased by 54% from late May to September. Klamerus-Iwan and Witek33 reported that the water storage capacity and wettability of oak (Quercus robur L.) canopy and the aromatic hydrocarbon in leaves increased from May to September. Therefore, a deep understanding of the seasonal changes in throughfall DOM characteristics and chlorine reactivity would help optimize the treatment processes in drinking water facilities to ensure water safety.34,35 Leaf extraction was sometimes used as an easy approach to simulate the throughfall chemistry.36,37 However, it is not sure whether leaf-extract DOM could be used to understand the characteristics and chlorine reactivity of throughfall DOM across different seasons. If the leaf-extract DOM could be a good proxy for throughfall DOM, leaves of various tree species could be rapidly tested to examine the characteristics of their throughfall DOM without requirement of real rainfall events. Here, loblolly pine (Pinus taeda L.) and turkey oak (Quercus cerris L.), one representative


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evergreen and one representative deciduous tree species in the southeastern US, were studied. We collected their throughfall samples during the four seasons and prepared leaf-extract samples.This study aimed to (1) determine the seasonal variations in DOM concentration [indicated by the dissolved organic carbon (DOC) level] from different canopies; (2) characterize the optical properties and chlorine reactivity of throughfall DOM and evaluate whether the seasonal variations are forest-type-specific; and (3) examine whether leaf-derived DOM we extracted could be a good proxy for throughfall DOM. Moreover, the DOC yields from the throughfall and runoff of pine and oak forests in South Carolina, US were estimated to explore the potential impact of throughfall DOM as a DBP precursor in forested watersheds.

2. MATERIALS AND METHODS 2.1. Sample Collection The experiment was conducted at Hobcaw Barony in North Winyah Bay, South Carolina, US. According to the US climate data (www.usclimatedata.com), the local mean annual precipitation is 1429 mm, and the mean annual temperature is 18 °C. In this area, loblolly pine (Pinus taeda L.) and turkey oak (Quercus cerris L.) are the common evergreen and deciduous species, respectively. Three healthy loblolly pine trees and three healthy turkey oak trees in the woodland (33o21’41’’N, 79o13’25’’W) near the Belle W. Baruch Institute of Coastal Ecology and Forest Science were selected. All six trees were mature (>10 years old), with a height greater than 4 m and a diameter at breast height that exceeded 10 cm. None of the trees hosted epiphyte cover, which is likely to alter the amount and chemistry of throughfall.20,22 Each tree was approximately 30-50 meters away from the closest neighboring tree. To collect the throughfall, three precombusted aluminum trays with a diameter of 25 cm were placed beneath the canopy hours before



a rainfall event. After a rainfall event (each event >20 mm and lasting more than 4 hours), the throughfall in trays was immediately transferred to the laboratory. All throughfall samples were filtered through 0.45-m polyethersulfone membranes, and then stored at 4 °C in clean amber bottles without headspace before analyses. The throughfall samples were collected in December 2014 (winter), April 2015 (spring), August 2015 (summer), and October 2015 (fall), and there was no rainfall for at least two weeks prior to the sample collection. In parallel with the throughfall collection, three rainwater samples were also collect ed during the same rainfall events over an open lawn that was approximately hundreds of meters away from the woodland. To explore the seasonal variation of leaf-derived DOM (leaf-extract DOM) without the interference of rainfall, fresh pine needles and oak leaves were collected from the same trees during the same months approximately one week before the rainfall events. Six locations on each tree were randomly selected for leaf collection. The fresh leaf samples were immediately extracted using a fixed leafto-water extraction ratio for leaf-extract DOM. Specifically, fresh leaves was extracted using a 5 g to 200 mL of leaf to water ratio in a 500-mL Erlenmeyer flask on a shaker at 200 rpm for 2 hours. After passing through a 0.45-m polyethersulfone membrane, the leaf extract was stored at 4 °C in a clean bottle without headspace before analyses. The water content of leaves was measured by oven drying the fresh leaf samples at 50 °C for 48 hours.

2.2. Chemical Analyses All filtered rainfall, throughfall, and leaf extract samples were analyzed by a TOC/TN analyzer (Shimadzu, Japan) to determine the concentrations of DOC and total dissolved nitrogen (TDN).38 The samples were also analyzed by UV-visible spectrometry (UV-1800, Shimadzu, Japan) and 3D spectrofluorometry (RF5301, Shimadzu, Japan).39 The specific UV absorbance at


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254 nm (SUVA254; in L/mg-C/m) is an indicator of DOM aromaticity and was calculated by dividing the UV absorbance at 254 nm by the DOC concentration.40 The E2/E3 ratio was calculated by dividing the absorbance at 254 nm by that at 365 nm.41 The raw fluorescence excitation−emission matrices (EEMs) were corrected for instrument-dependent effects, inner-filter effects, and Raman effects and then standardized to Raman units.42 The fluorescence index (FI) is an index used to differentiate terrestrial and microbial origins of DOM and was determined as the ratio of fluorescence intensity at emission wavelengths (Ems) of 470 nm and 520 nm at excitation wavelength (Ex) of 370 nm.43 The freshness index (β/α) was determined as the ratio of the fluorescence signal at Em of 380 nm to the maximum signal between Ems of 420 and 435 nm at Ex of 310 nm.44 The humification index (HIX) was calculated by dividing the peak area under Em range of 435−480 nm by the peak area under Em range of 300−345 nm at Ex of 254 nm.45 Through fluorescence regional integration based on Simpson’s rule, EEMs were operationally divided into five regions, including I) tyrosine-like, II) tryptophan-like, III) fulvic acid-like, IV) soluble microbial byproduct-like, and V) humic acid-like.46,47 We also calculated the percent fluorescence response in each region (Pi,n).46 A chlorination test was used to evaluate the DBP formation potential (DBP-FP) and chlorine reactivity of DOM.48 The filtered samples were diluted, buffered to pH 8.0, and reacted with freshly prepared NaOCl/H3BO3 solution at 25 °C and pH of 8 in the dark for 24 h as described in Wang et al.49 The reaction was then stopped by adding small amount of 10% Na2SO3 solution. The resulting solution was saturated by Na2SO4 and the DBPs were then extracted with MTBE. DBPs in the MTBE solution were analyzed by an Agilent 7890 gas chromatography – electron capture detector using the EPA method 551.1. The analytes included four types of THMs (trichloromethane, dichlorobromomethane, dibromochloromethane, and tribromomethane), four



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types of HANs (trichloroacetonitrile, dichloroacetonitrile, bromochloroacetonitrile, and dibromoacetonitrile), three types of haloketones (HKTs; 1,1-dichloro-2-propanone, 1,1,1trichloro-2-propanone, 1,2,3-trichloropropanone), and chloral hydrate (CHD). The minimum reporting levels of the present study were about 0.1-0.3 μg/L. We also calculated the specific DBPFP (SDBP-FP) in the unit of μg-DBP/mg-DOC as the ratio of the DBP concentration to the DOC concentration. The SDBP-FP is an indicator of the DOM reactivity in forming DBPs.

2.3. DOC Yields from Throughfall and Runoff We made a rough estimation of the DOC yields in throughfall and runoff from the pine and oak canopies at 34 watersheds in South Carolina based on the assumption that the measured data here could be extrapolated (Eqs. 1-3; Figure S1). The surface runoff volume was estimated by Eqs. 4&5.50 The daily rainfall between December 1, 2014 and November 31, 2015 for a total of 168 stations in South Carolina was retrieved from Climate Data Online managed by the National Oceanic and Atmospheric Administration (https://www.ncdc.noaa.gov/cdo-web/) (Figure S2). We employed the ordinary kriging method with a 30-m spatial resolution for spatial rainfall interpolation, and the performance was evaluated by the mean absolute error as well as the root mean square error (Table S1). Land cover data were obtained from the 2014 cropland data layer at 30-m resolution (https://www.nass.usda.gov/Research_and_Science/Cropland/SARS1a.php), and the regions of evergreen and deciduous forests were considered as the distribution of pine and oak trees because of their dominance (https://data.fs.usda.gov/geodata/rastergateway/forest_type/) (Figure S3). The hydrologic soil group data were obtained from the STATSGO dataset managed by

the

National

Cooperative

Soil

Survey

(https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/survey/geo/?cid=nrcs142p2_053629)


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(Figure S4). 𝑌𝑡ℎ𝑟𝑜𝑢𝑔ℎ𝑓𝑎𝑙𝑙 =

∑𝑖∑𝑘(𝑉𝑡ℎ𝑟𝑜𝑢𝑔ℎ𝑓𝑎𝑙𝑙,𝑖,𝑘 × 𝐷𝑂𝐶throughfall)

𝑌𝑟𝑢𝑛𝑜𝑓𝑓 =

𝑛×𝐴 ∑𝑖∑𝑘(𝑄𝑟𝑢𝑛𝑜𝑓𝑓,

𝑖,𝑘

× 𝐷𝑂𝐶runoff )

𝑛×𝐴

𝑉𝑡ℎ𝑟𝑜𝑢𝑔ℎ𝑓𝑎𝑙𝑙,𝑖,𝑘 = 10 × 𝐴 × (𝛼0 × 𝑃𝑖,𝑘 + 𝛽0 )

𝑉𝑟𝑢𝑛𝑜𝑓𝑓,𝑖,𝑘 = 10 × 𝐴 ×

𝑆𝑘 = 25.4 × (

(𝑃𝑖,𝑘 ― 𝐼𝑎,𝑘)2 𝑃𝑖,𝑘 ― 𝐼𝑎,𝑘 + 𝑆𝑘

1000 ― 10) 𝐶𝑁𝑘

Eq. (1)

Eq. (2) Eq. (3) Eq. (4)

Eq. (5)

where Ythroughfall is the DOC yield from the pine or oak canopy that reaches the forest floor via throughfall, kg-C/ha; Yrunoff is the runoff DOC yield from the pine or oak canopy, kg-C/ha; i indicates the ith rain event; k is the kth grid cell; A is the area of a grid cell (30×30 m2 = 0.09 ha), ha; n is the number of grid cells occupied by pine or oak forests in each watershed in South Carolina; DOCthroughfall is the DOC concentration in throughfall, kg/m3; DOCrunoff is the DOC concentration in runoff and is assumed to be equal to DOCthroughfall, kg/m3; Vthroughfall,i,k is the volume of throughfall during the ith rain event in the kth grid cell; m3; α0 and β0 are the empirical constants to estimate throughfall from gross rainfall as determined by regression analysis [α0=0.88 and β0=0.8 (mm) for pine51 and α0=0.87 and β0=0.35 (mm) for oak20]; Pi,k is the gross rainfall depth of the ith rainfall event in the kth grid cell, mm; Vrunoff,i,k is the runoff volume from the ith rainfall event in the kth grid cell, m3; Iα,k is the initial abstraction including surface storage, interception, and infiltration (approximated as 0.2Sk), mm; Sk is the soil water retention parameter in the kth grid cell; and CNk is the curve number in the kth grid cell, according to the USDA Soil Conservation Service.52



2.4. Data Analyses RStudio Desktop v1.0.44 (Boston, MA, US) was used for statistical analyses. A paired ttest (using the t.test function in the stats package) or Wilson test (while the difference was not normally distributed; using the wilcox.test function in the stats package) was conducted to test the difference between groups of DOM sources and seasons. The Shapiro-Wilk normality test (using the shapiro.test function in the stats package) was conducted to test the normality. After obtaining the p values, the orderPvalue function in the agricolae package was used to group the average values. For correlation analysis, correlation coefficients and p values were calculated by the cor and cor.test functions, respectively, in the stats package. Factor analysis of optical properties and SDBP-FPs was performed using the fa function (nfactors=2, rotate="varimax", SMC=FALSE, fm="minres") in the psych package. To evaluate the greatness of seasonal variations, the ratio of maximum and minimum values among the four seasons was calculated for each parameter.

3. RESULTS AND DISCUSSION 3.1. DOC and TDN Concentrations The DOC concentration (mean±standard deviation; in mg-C/L) was lowest in rainwater (1.2±0.4), followed by pine (26.0±19.7) and oak throughfall (38.8±37.8), and then pine-leaf (162±230) and oak-leaf extracts (323±311; Table 1). The DOC concentrations in throughfall measured here were at the high end of the wide concentration range (5.7-54.0 mg-C/L) of forest throughfall reported in previous studies (Table S2), indicating that pine and oak canopies are important sources of throughfall DOM. We also observed the high spatial variability of the throughfall DOC concentrations in our study,53 suggesting that higher number of repeated samples


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would be favorable to better quantify the DOM fluxes. The reason for the high spatial variability in our throughfall DOC could be that the water samples were collected from the areas beneath canopies with varied density. The throughfall DOC could be lower when the throughfall samples had less canopy interaction. Although studies on the spatial variability of throughfall DOC concentration for a single tree are rare, it is generally assumed that the characters of throughfall DOM do not vary greatly for a single species without epiphytes.20,21 The DOC concentrations in leaf extracts were significantly and consistently higher than those in the corresponding throughfall samples, probably because of the relatively high leaf-to-water ratio and/or the intensive physical strength exerted during extraction. Similarly, the TDN concentration (in mg-N/L) increased in the order of rainwater (0.35±0.09) < oak throughfall (0.55±0.18) ≈ pine throughfall (0.65±0.34) < pine-leaf extracts (2.00±2.14) < oak-leaf extracts (3.57±3.37). The large differences in DOC and TDN concentrations between throughfall and rainfall support that significant amounts of carbon and nitrogen could be leached from tree canopies.20,21,54-57 The DOC/TDN ratio was ranked as rainwater (4.01±2.37) < pine throughfall (38.5±14.6) ≈ pine-leaf extracts (59.9±27.8) ≈ oak throughfall (71.3±63.1) < oak-leaf extracts (151±134). The DOC/TDN ratios in both throughfall and leaf extracts were higher than those in previous studies (i.e., 9-44 in temperate forests58 and 16-21 in rainforests59) and significantly exceeded the stoichiometry of the microbial requirement for biomass building.20 The concentrations of DOC and TDN and the DOC/TDN ratio showed wide seasonal variations, and such seasonal variations also depended on plant species (Figure 1). The maximum to minimum DOC ratio across the four sampling periods (DOCmax/min) was 4.27 (maximum in fall/minimum in winter) in pine throughfall, 6.87 (spring/summer) in oak throughfall, 18.6 (fall/spring) in pine-leaf extracts, and 9.18 (fall/summer) in oak-leaf extracts (Figure 1). The



TDNmax/min was 2.63 (fall/summer) in pine throughfall, 1.91 (fall/summer) in oak throughfall, 10.4 (fall/spring) in pine-leaf extracts, and 15.6 (fall/spring) in oak-leaf extracts. The highest DOC and TDN concentrations were found in fall for pine throughfall, pine-leaf extracts, and oak-leaf extracts. This result may be a consequence of the leaf aging in canopy. The wettability of oak leaf surface has been found higher in September than May,33 which may allow more organics leaching from the leaves. In contrast, oak throughfall had the highest DOC concentration in spring, which is in agreement with findings in other oak-dominated areas.60,61 This result implies that sources other than leaves may have contributed to the DOC concentrations in the spring oak throughfall. We observed that oak trees produced a large amount of pollen during spring, which could have strongly contributed to the highest DOC concentration measured in oak throughfall in spring.20,62 The (DOC/TDN)max/min was 2.21 (spring/winter) in pine throughfall, 5.71 (spring/fall) in oak throughfall, 3.22 (fall/winter) in pine-leaf extracts, and 8.84 (summer/winter) in oak-leaf extracts. The DOCmax/min, TDNmax/min, and (DOC/TDN)max/min values were higher in leaf-extract DOM than in throughfall DOM, and higher values were measured in the oak compared with the pine sources (Figure 1). This suggests that leaf-extract DOM was a highly season-sensitive contributor to the throughfall DOM pool, and there might be lower seasonal variations in DOM chemistry associated with pine compared with oak.

3.2. DOM Optical Properties SUVA254 (L/mg-C/m) was significantly lower in pine-leaf extracts (0.50±0.18) than in oakleaf extracts (2.88±2.13) and in pine throughfall (1.63±0.34) than in oak throughfall (3.11±1.63) (p0.05) among all five fluorescent regions, suggesting the insignificant influence of tree species on regional


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fluorescence responses of throughfall DOM. Compared to throughfall, the leaf-extract DOM had higher PI,n (tyrosine-like) and PII,n (tryptophan-like) and lower PIII,n (fulvic acid-like) and PV,n (humic acid-like) for both pine and oak (p

Throughfall Dissolved Organic Matter as a Terrestrial Disinfection

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