Herbicide Concentrations in First-Order Streams after Routine ...

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S. Lynsey Scarbrough, C. Rhett Jackson, Samantha Marchman, Ginny Allen, Jeff Louch, and. Masato Miwa. Herbicides are an important tool for managing ...
For. Sci. 61(3):604 – 612 http://dx.doi.org/10.5849/forsci.14-051 Copyright © 2015 Society of American Foresters

APPLIED RESEARCH

soils & hydrology

Herbicide Concentrations in First-Order Streams after Routine Application for Competition Control in Establishing Pine Plantations S. Lynsey Scarbrough, C. Rhett Jackson, Samantha Marchman, Ginny Allen, Jeff Louch, and Masato Miwa Herbicides are an important tool for managing competitive vegetation in pine silviculture, and forestry best management practices (BMPs) were designed partly to minimize the movement of overland flow and dissolved herbicides into adjacent streams. We measured herbicide concentrations in streams before and after application when all modern forestry BMPs were applied to silvicultural operations. Imazapyr, hexazinone, and sulfometuron methyl were applied operationally to pine plantations covering 45 and 54% of the watershed area of two first-order streams in the Upper Coastal Plain of Georgia, USA. Herbicides in stream water were sampled and analyzed before and for several months after application. All three herbicides were detected during stormflows after application, but the highest observed concentration was 7.7 parts per billion (ppb), just a few ppb above the level of quantification. The highest concentrations occurred in the first or second stormflow event after application, and peak concentrations diminished rapidly in subsequent events. Quantifiable concentrations occurred as pulses lasting 1/2–1 day after the hydrograph peak. Herbicide concentrations were below or near the level of quantification for all baseflow samples. Our results suggest that transport of these operational silvicultural herbicides to streams is low with proper application and use of modern forestry BMPs, and these results are in concurrence with other recent studies of herbicide movement from modern forestry operations. Keywords: best management practices (BMPs), silviculture, herbicides, water quality, hydrology

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o reduce competition in pine plantations, herbicides typically are applied 1–3 times during each rotation (Michael 2004), with pine rotations in the southeastern United States lasting from 18 to 30 years. Generally, herbicides are applied as chemical site preparation before planting, as herbaceous weed control a few months after planting, and finally as a means to release crop trees from woody competition 2– 4 years after planting (Shepard et al. 2004). All forestry herbicides are water soluble to some degree so their movement is strongly influenced by the amount of water available to facilitate transport (Michael and Neary 1991). The primary cause of stream herbicide contamination is either direct application of herbicides to

flowing channels, spray drift, or transport by overland flow (Michael and Neary 1991). However, forestry herbicides bond strongly to organic matter and soils, so if they have time to bond before rainfall occurs, very little herbicide will be mobilized. For this reason, forestry best management practices (BMPs) call for applying herbicides during dry conditions when the wind is not blowing hard (to avoid spray drift). Forestry BMPs are designed to minimize the production of Hortonian overland flow and to impede the movement of overland flow from roads, landings, and plantation areas to streams. BMPs include placing roads and landings far from streams, dispersing road runoff, avoiding stream crossings, and minimizing soil compaction. The

Manuscript received March 24, 2014; accepted November 19, 2014; published online December 31, 2014. Affiliations: S. Lynsey Scarbrough ([email protected]), University of Georgia. C. Rhett Jackson ([email protected]), University of Georgia, Warnell School of Forest Resources, Athens, GA. Samantha Marchman ([email protected]), Plum Creek Timber. Ginny Allen ([email protected]), National Council for Air and Stream Improvement. Jeff Louch ([email protected]), National Council for Air and Stream Improvement. Masato Miwa ([email protected]), Fukuoka University. Acknowledgments: This research was funded in part by the NCASI, Inc., International Paper Company, the J.W. Jones Ecological Research Center, and the Georgia Forestry Commission. Scott Terrell, David Jones, William Summer, Michael Bell, Andrew Morrison, and Andrew Mishler conducted the field data collection and assisted with data analysis. Steve Golladay provided valuable advice on sampling and analysis. This article uses metric units; the applicable conversion factors are: meters (m): 1 m ⫽ 3.3 ft; square meters (m2): 1 m2 ⫽ 10.8 ft2; kilometers (km): 1 km ⫽ 0.6 mi; millimeters (mm): 1 mm ⫽ 0.039 in.; milliliters (mL): 1 mL ⫽ 0.061 in2 (dry) ⫽ 0.27 fluid dram (liquid); liters (L): 1 l ⫽ 61.02 in2, ⫽ 0.908 quart (dry), ⫽ 1.057 quart (liquid); hectares (ha): 1 ha ⫽ 2.47 ac. 604

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creation of streamside management zones (SMZs) with widths prescribed by stream characteristics and adjacent slope gradients is one of the BMPs used to reinfiltrate surface runoff and sequester entrained pollutants. However, variable source areas such as ephemeral channels and swales may not be fully protected if they extend outside the SMZ (Neary et al. 1983, Michael et al. 1999, Rivenbark and Jackson 2004). In such cases, herbicides may be directly applied to ephemeral channels and subsequently transported into active channels during wet periods (Michael and Neary 1991, Michael et al. 1999). Whereas SMZs have been shown to successfully sequester sediments and nutrients (Ward and Jackson 2004, White et al. 2007), the extent to which SMZs attenuate herbicide movement to streams is not well understood. For example, a 10-m-wide forested SMZ attenuated only 49% of atrazine and 5% of picloram in artificial runoff moving in shallow overland flow (Pinho et al. 2008), underscoring the importance of herbicide sorption to soils and organic matter before heavy rainfall occurs. Herbicide movement and persistence within the soil profile is dependent on the herbicide applied, the soil properties (particular organic matter content and pH), the climatic conditions, and the timing of the first postapplication rain event (Michael and Neary 1991, 1993, McBroom et al. 2013). Asmussen et al. (1977) and Arora et al. (1996) concluded that dry antecedent conditions increased herbicide infiltration and retention and that adsorption to soil and/or uptake by vegetation also increased retention. Some herbicides have intermediate adsorption (0.1 ⬍ K ⬍ 100) and remain in the dissolved phase, whereas others are tightly adsorbed to soils (K ⬎100) and are transported with sediments (Fawcett et al. 1994). Regardless, the presence of vegetated buffer strips reduces transport of herbicides both dissolved in runoff and adsorbed to sediments by reducing flow velocity and allowing for greater infiltration and retention by the soil (Mersie et al. 1999, Seybold et al. 2001, Arora et al. 2003). Infiltration of surface runoff passes water and herbicides through the root zone where greater chemical and biological degradation may occur (Michael and Neary 1993, Michael 2004). Soils with greater clay and organic matter contents retain and degrade herbicide molecules more efficiently (Michael and Neary 1993). Soil and water acidity also affects the rate of degradation of certain herbicides (Michael 2003). Herbicides tend to not move great distances laterally through the soil profile, with the exception of transport through macropores (Michael and Neary 1993, Michael 2004). Timing of the first postapplication rain event is particularly important since the first two to three storm events have been shown to be responsible for approximately 90% of all herbicide transport to streams (Michael and Neary 1991, Lowrance et al. 1997, Patty et al. 1997). It has been reported that herbicide transport during these stormflows can occur as short pulses lasting a few hours (National Council for Air and Stream Improvement [NCASI] 2013). As time increases between herbicide application and the first storm event, transport to nearby streams decreases (Michael and Neary 1991, McBroom et al. 2013). The high costs of herbicide sampling and analysis coupled with the short duration of herbicide presence in streams after application has resulted in few published studies of herbicide residues in streams draining forest plantations with modern forestry BMPs implemented. Under modern BMPs of the last 15 years, only three such studies have been conducted in the United States: in Oregon (NCASI 2013), in East Texas (McBroom et al. 2013), and in a wetland system of the South Carolina Coastal Plain (Michael et al.

2006). Every watershed scale study is a case study, with differences in soils, topography, climate, dominant hydrologic processes, and the particulars of silvicultural activities and BMP implementation affecting the results. To obtain robust knowledge about herbicide behavior in forest plantations, we need multiple studies to quantify the variance in hydrologic behavior and water herbicide export and to seek surprise results. The purpose of this study was to measure herbicide residue concentrations after operational applications to pine plantations established with full implementation of modern forestry BMPs. This study was part of the Dry Creek paired watershed study designed to evaluate holistically the effectiveness of the Georgia Forestry Commission’s 1999 recommended BMPs in a relatively steep southeastern Coastal Plain environment (Terrell et al. 2012, Marchman et al. 2015). Harvest and planting of the study watersheds resulted in a large increase in water yield, a rise in the water table, and the appearance of toeslope seeps (Terrell et al. 2012). Sediment concentrations were unaffected by harvest (Terrell et al. 2012), but nitrate and total nitrogen concentrations increased slightly (Marchman et al. 2015). As a result of the increased streamflow, sediment and nitrogen yields increased after harvest, but increases were small compared to interannual and among-watershed variability. These studies demonstrated that implementation of modern forestry BMPs allowed only minor water quality changes but did not achieve a zero-impact standard (Jackson 2014).

Methods Study Location

This study focused on two adjacent first-order watersheds that drain into Dry Creek, a tributary to Lake Seminole approximately 12 km southwest of Bainbridge in Decatur County (84°37⬘30⬙ W, 30°47⬘30⬙ N) in the upper coastal plain of southwestern Georgia. The watersheds were owned at the time of the study by International Paper and designated by the company as Southlands Forest. The T1 and T2 watersheds drain 33.3 and 44.1 ha, respectively, and the average widths of the channels are 1.1 and 1.7 m (Terrell et al. 2012). Before harvest, forest stands on the watersheds were mature planted loblolly pine (Pinus taeda), and riparian areas were mixed pine hardwoods dominated by yellow-poplar (Liriodendron tulipifera) and swamp tupelo (Nyssa silvatica). Ultisols and Alfisols are the dominant soil orders. Riparian soils, mostly Chiefland and Esto series, feature well-drained fine sands over clay loams. Toeslopes consist of somewhat excessively well-drained Eustis series soils, with loamy sands over sandy loams, and uplands feature a mix of Wagram, Norfolk, Lakelan, Orangeburg, and Lucy series, mostly featuring well-drained loamy sands over sandy clay loams (Terrell et al. 2012). The hydrologic system is groundwater dominated, and the carbonate Upper Floridan Aquifer is ⬎20 m below the streambed. The two treatment streams differed in morphology, with one stream moderately incised into a V-shaped valley and the other stream weakly defined within a riparian wetland system. Because of these differences in valley and stream morphology, we viewed these two streams as providing separate case studies rather than the replication of one case study. Site Preparation

Between September and November 2003, 45% of T1 and 54% of T2 were clearcut harvested (Terrell et al. 2012) using all modern forestry BMPs (Georgia Forestry Commission 1999). Roads and landings were placed on ridgetops far from streams, runoff from Forest Science • June 2015

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roads and landings was dispersed to slopes, no stream crossings were implemented, and SMZ boundaries were taped and marked at the widths recommended by the 1999 Georgia Forestry BMP manual. The upstream half of each watershed’s SMZ was not harvested, whereas the downstream half was partially harvested at 11.5 m2 of residual basal area per ha to see whether riparian thinning affected sediment and nutrient concentrations in streams (Terrell et al. 2012, Marchman et al. 2015). SMZ widths were 12 m (40 ft) on slopes less than 20% and 21 m (70 ft) on slopes of 21– 40%. Herbicide Application and Sampling

The herbicides applied and monitored in this study are commonly used in the Southeast for competition control during establishment of pine plantations. Imazapyr is a systemic herbicide that inhibits normal plant growth by preventing synthesis of branchedchain amino acids (Environmental Protection Agency [EPA] 2006). It is nonvolatile and both persistent and mobile in soil. It is only known to degrade via photolysis and has a half life of 3–5 days in surface water and 25–142 days in soil (Tatum 2004, EPA 2006). Hexazinone is a contact herbicide that blocks photosynthesis within the chloroplasts (Ganapathy 1996). It is weakly adsorbed in most soils, is very soluble in water, and has a moderate to long half-life (Ganapathy 1996, Tatum 2004). Hexazinone degrades via biological decomposition and photodegradation (Ganapathy 1996), with a half-life in soils ranging from 24 days to 1 year depending on the local climate conditions (Michael et al. 1999). Sulfometuron methyl is a systemic herbicide that acts to hinder plant cell growth by inhibiting the production of amino acids (EPA 2008). It does not sorb strongly to soils and is degraded via abiotic and microbially mediated hydrolysis under acidic conditions as well as photolysis (Michael 2003, EPA 2008). The half-lives of sulfometuron methyl in water range from 1 to 18 days and in soil from 5 to 65 days (Michael 2003). Chopper (active ingredient: imazapyr, 27.6%) was applied for site preparation on Sept. 1, 2004, per label rates and guidance at 0.38 L ha⫺1 with a John Deere 540 skidder in the uplands and with a backpack sprayer on the slopes. Subsequently, on Sept. 30, 2004, Chopper was reapplied by backpack to 12 ha of sloped areas adjacent to SMZs. On Nov. 15 and 16, 2004, a controlled burn was done on the two treatment watersheds. In December 2004, the treatment sites were replanted with loblolly pine. To control herbaceous vegetation, Oustar (active ingredients: hexazinone, 63.2%; and sulfometuron methyl, 11.8%) was applied on Mar. 15, 2005, per label rates and guidance at 138 g ha⫺1 with a backpack sprayer. These application rates were higher than those typically used by the landowner, International Paper, but the higher label rates were used in this study to produce the herbicide runoff potential associated with the label rates. Application was done by an herbicide contractor under the supervision of a contract registered forester. Chemicals were mixed on the ridgetop roads far from streams, and herbicides were applied under dry conditions, per BMP guidance. Baseflow grab samples and storm event samples were collected from Sept. 3 through Nov. 30, 2004, and from Mar. 16 to Apr. 15, 2005. Before herbicide application, baseflow grab samples were collected from Aug. 20 to 30, 2004. Both streams were instrumented with a 229-mm (9-in.) fiberglass Parshall flume (Tracom, Inc., Atlanta, GA) to measure stream discharge. An autosampler (ISCO model 3700 with 1-liter polypropylene bottles) was installed in each watershed to collect water samples during storm events. 606

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All samples were preserved at pH 7 at the time of collection by adding 5 mL of a 2 M phosphate buffer solution to all autosampler bottles before deployment and to the manual grab samples in the field at the time of collection. All autosampler bottles were retrieved within 24 hours of collection of each storm event’s first sample and taken to the hydrology laboratory on Southlands Forest. All samples were frozen for storage and shipped by freezer truck (Agricultural Chemicals Development Services, Inc., Phelps, NY) to the NCASI West Coast Regional Center analytical laboratory in Corvallis, OR, for analysis. Quality Assurance/Quality Control and Analysis

Field quality assurance (QA) samples were generated at Southlands Hydrology. These consisted of field blanks, field blank spikes, and sample field spikes (sample splits fortified with herbicides). All field blanks and blank spikes were prepared using deionized and carbon-filtered water generated at the field laboratory. All field blank spikes and field sample spikes were performed at approximately 5 ppb (depending on exact sample volume) by adding 50 ␮l (glass syringe; VWR catalog no. 60376-241) of a spike solution provided by NCASI and stored in a freezer when not in use. Dissolved concentrations of imazapyr, hexazinone, and sulfometuron methyl were determined using NCASI method HIMS-W106.01 (NCASI 2007). The procedure consists of solidphase extraction of 200 mL of filtered (0.45 ␮m) stream water samples, concentration to 1 mL final extract volume, and analysis by high-performance liquid chromatography with UV detection. Imazapyr concentrations are reported as acid equivalent (a.e.) and hexazinone and sulfometuron methyl concentrations are reported as active ingredient (a.i.). The laboratory method detection limits (MDL) and limits of quantification (LOQ) for all three herbicides are found in Table 1. In addition to the analysis of field QA samples, laboratory QA/quality control (QC) included generation of multipoint calibration curves and ongoing analysis of calibration verification standards, method blanks (reagent water blanks), and spiked blanks. In addition, most analytical batches included a matrix spike and matrix spike duplicate experiment. Interpretation of Data and Time Series

Trends in stormflow herbicide concentrations were assessed by plotting all measured concentrations with the hydrographs over the period of monitoring. Because the highest concentrations for herbicides are often observed in the first or second storm after application, descriptive statistics of the distribution of concentrations observed during these storms were calculated. In addition, the variations of herbicide concentrations through individual storms were plotted and assessed.

Results Results (not shown) from laboratory QA/QC analyses showed that background contamination from the laboratory had no impact on sample results and also showed that the Dry Creek sample matrix exerted no notable matrix effects on the analysis. Field blank results (not shown) showed that background contamination from the sampling itself was not a factor. Most importantly, recoveries of the prefreeze sample (field) spikes were uniformly good (Table 1), showing that the analysis effectively gave quantitative recovery of all three herbicides at nominally 5 ppb while also documenting stability of all

Table 1. Application dates, detection limits, LOQ, field spike recoveries, and concentration distributions for the first stormflow event after application of each herbicide. Imazapyr Application date MDL (ppb) LOQ (ppb) Field (sample) spike results (% recovery)c Treatment watershed Date of storm with maximum concentration Maximum concentration (ppb) 75th percentile (ppb) 50th percentile (ppb) 25th percentile (ppb) Minimum concentration (ppb)

a

b

Sept. 1, 2004 ; Sept. 30, 2004 0.30 0.97 104 ⫾ 8.9 (n ⫽ 24) T1 T2 Sept. 7, 2004 Sept. 16, 2004 7.30 2.60 4.63 1.33 1.60 0.68 0.51 0.14 ND ND

Hexazinone

Sulfometuron methyl

Mar. 15, 2005 0.30 0.97 103 ⫾ 5.2 (n ⫽ 20) T1 T2 Mar. 16, 2005 Mar. 16, 2005 7.50 7.70 4.40 4.90 3.30 4.40 1.85 3.80 ND 2.10

Mar. 15, 2005 0.17 0.55 98 ⫾ 6.5 (n ⫽ 24) T1 T2 Mar. 16, 2005 Mar. 16, 2005 0.99 1.24 0.57 0.67 0.37 0.54 0.23 0.42 ND 0.21

All imazapyr concentrations are as acid equivalents (a.e.), whereas all hexazinone and sulfometuron methyl concentrations are as active ingredient (a.i.). ND, not detectible; ppb, parts per billion. a Applied near SMZs. b Applied on slopes. c All spikes were added to whole samples before to freezing; all spikes were nominally 5 ppb.

Figure 1. Imazapyr (baseflow and stormflow) concentrations and mean daily flows in T1 and T2. Concentrations below the MDL (