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UNXTED STATES ENVJRONMENTAL PROTECTION AGEN . REGION 8 I 1595 WYNKOOP I AUG o DENVER,CO 80202-1129 Phone 800-i27-8917 w E R ~ u WNoNl m ~ ~ir0~5)iv,s1a1, h~:/lwww.~a.~ovl&on08 a

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JUL 3 0 2089 ti1

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Ref: ,SEPR-EP Mr. &eve-Gunderson

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Water Quality Control Division Colof-adoDepartment of Public Health and Environment 4300 Cherry Creek Drive South DenveryColorado 80246-1 53 0

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MDL Approvals rumble Gulch - Segment COSPB004a u,

Dear )Mr.Gunderson:

zn,pH daily loads (TMDLs) as submitted by your In accordance with the Clem Water Act developed for certain pollutants in water Based on our review, we feel the separate are adequately addressed, taking into

1 We have completed our review of the offid for the waterbodies listed in the enclosure to this (33 U.S.C. 125 1 et. seq.), we approve all aspects of quality limited watehcdies as described in Section TMIX elements for the pollutants listed in the cons1 eration seasonal variation and a margin of safety.

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iew and approval. If you have any questions, and she m y be reached at (303) 3 12-6947.

k k you for submitting fhese TMDLs for w the mpst lmowkdgeable person on my staff is Sandra

Carol L. Campbell ~ s s i d nRegional t Administrator Ofice of Ecosystems Protection and Remediation

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TOTAL MAXIMUM DAILY LOAD ASSESSMENT GAMBLE GULCH COSPBO04A Copper, Zinc, and pH GILPIN COUNTY, COLORADO April 2009

Waterbody Description / WBID Pollutants Addressed Relevant Portion of Segment (as applicable) Use Classifications / Designation Water Quality Target

TMDL Goal

TMDL Summary Mainstem of South Boulder Creek, including all tributaries, lakes, reservoirs, and wetlands, from the source to the outlet with Gross Reservoir, COSPBO04a Dissolved copper, Dissolved zinc, pH Gamble Gulch

Aquatic Life Cold 1, Recreation 1a, Water Supply, Agriculture;

Segment 4a Cu-D

Chronic TVS=e0.8545[ln(hardness)]-1.7428)

Acute TVS

e 0.9422[ln(hardness)]-1.7408)

Zn-D TVS=e0.8473[ln(hardness)]+0.8618) TVS=e0.8473[ln(hardness)]+0.8699) (pre2010) Zn-D TVS=0.986e0.8525[ln(hardness)]+0.9109) TVS=0.978e0.8525[ln(hardness)]+1.0617) (2010) pH 6.5-9.0 6.5-9.0 Attainment of Aquatic Life use classification standards for Cu, Zn, and pH.

EXECUTIVE SUMMARY Boulder Creek Segment 4a, Gamble Gulch, has been on the State’s 303(d) list of water-quality impaired waterbodies for nonattainment of water quality standards for pH, dissolved copper and dissolved zinc since 2006, where it was given a high priority (Table 1). These metals impair the Aquatic Life Cold 1 classification for Segment 4a. The high concentration of metals is primarily the result of mining activity in the watershed since the1850’s. Gamble Gulch is located below the inactive Tip Top Mine near Rollinsville in Gilpin County, Colorado. Water quality above the Tip Top Mine is relatively pristine and in attainment of assigned standards while water quality below the mine has elevated copper and zinc levels and associated low pH. The low pH exceeds water quality standards for aquatic life uses as well as recreation 1a uses. Acid mine drainage directly enters Gamble Gulch.

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Segment # Segment Description Portion Segment 4a Mainstem of South Boulder Creek, including all Gamble tributaries, lakes, reservoirs, and wetlands, from the Gulch source to the outlet with Gross Reservoir, COSPBO04a

303(d) Listed Contaminants Cu, Zn, pH

Table 1. Segment within the Boulder Creek watershed that appears on the 2006 and 2008 303(d) list of impaired water bodies.

II. INTRODUCTION Section 303(d) of the federal Clean Water Act (CWA) requires states to periodically submit to the U.S. Environmental Protection Agency (EPA) a list of water bodies that are water-quality impaired. Water-quality limited segments are those water bodies that, for one or more assigned use classifications or standards, the classification or standard is not fully achieved. This list of water bodies is referred to as the “303(d) List”. In Colorado, the agency responsible for developing the 303(d) list is the Water Quality Control Division (WQCD). The List is adopted by the Water Quality Control Commission (WQCC) as Regulation No. 93. The WQCC adopted the current 303(d) list in March of 2008. For waterbodies and streams on the 303(d) list a Total Maximum Daily Load (TMDL) is used to determine the maximum amount of a pollutant that a water body may receive and still maintain water quality standards. The TMDL is the sum of the Waste Load Allocation (WLA), which is the load from point source discharge, Load Allocation (LA) which is the load attributed to natural background and/or non-point sources, and a Margin of Safety (MOS) (Equation 1). (Equation 1)

TMDL=WLA+LA+MOS

Alternatively, a segment or pollutant may be removed from the list if the applicable standard is attained, if implementation of clean-up activities via alternate means will result in attainment of standards, if the original listing decision is shown to be in error or if the standards have been changed as the result of a Use Attainability Analysis (UAA), or other EPA approved recalculation method. Gamble Gulch is a portion of Segment 4a (the mainstem of South Boulder Creek, including all tributaries, lakes, reservoirs, and wetlands, from the source to the outlet with Gross Reservoir) and is identified on the 2006 and 2008 303(d) list for exceeding the water quality standards for pH, dissolved copper and dissolved zinc (Table 1) (WQCC, 2006b). The impairment status for designated uses in Gamble Gulch is presented in Table 2.

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Final Date (Cycle Year) of Current Approved 303(d) list: 2008 WBID Segment Description Designated Uses & Impairment Status

COSPBO04a

Mainstem of South Boulder Creek, including all tributaries, lakes, reservoirs, and wetlands, from the source to the outlet with Gross Reservoir.

Aquatic Life Cold 1: Impaired Recreation 1a: Impaired Water Supply: Not Impaired Agriculture: Not Impaired

Table 2. Designated uses and impairment status for Segment 4a, Gamble Gulch.

Geographical Extent This listed portion of the South Platte Watershed is part of the South Platte Boulder Creek Basin, Hydrologic Unit Code (HUC) 10190005 and is located in Gilpin County. The upper basin of the Boulder Creek watershed, which includes South Boulder Creek and Gamble Gulch, is composed primarily of Precambrian siliceous metamorphic and granitic rocks. These rocks consist of gneisses and schists (1800 million years old) that were intruded by the Boulder Creek Granodiorite (1700 million years old) and the Silver Plume Granite (1400 million years old). In addition, early- and middle-Tertiary (30 to 60 million years old) deposits of metallic ores associated with intrusive dikes and sills are found in the upper basin. Deposits of gold, silver, tungsten, copper, lead, zinc, tin, and uranium were mined in the upper watershed beginning in 1859 (Lovering and Goddard, 1950; Bilodeau and others, 1987) (USGS, 2000). South Boulder Creek is part of the headwaters reach of the Boulder Creek watershed, and it receives transbasin water diverted from Denver Water’s collection systems in the Fraser and Williams Fork basins via the Moffat Tunnel. The headwater region is sparsely populated, but can be affected by recreation, air pollution, historical mining activity, road runoff, and mountain cabins (USGS, 2000). The drainage area of the South Boulder Creek watershed is 338 km2 and has a mean elevation of 2620 meters. The mean annual precipitation is approximately 582.9 mm. The headwaters region, of which Gamble Gulch is a component, is snowmelt dominated. Heavy metal pollution results from combination of both natural and anthropogenic sources, heavily dominated by acid mine drainage from the Tip Top Mine, an abandoned mine site. A map of the study area and associated sampling sites is shown in Figure 1.

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III.

WATER QUALITY STANDARDS

Standards Framework Waterbodies in Colorado are divided into discrete units or “segments”. The Colorado Basic Standards and Methodologies for Surface Water, Regulation 31(WQCC 2006b), discusses segmentation of waterbodies in terms of several broad considerations: 31.6(4)(b)…Segments may constitute a specified stretch of a river mainstem, a specific tributary, a specific lake or reservoir, or a generally defined grouping of waters within the basin (e.g., a specific mainstem segment and all tributaries flowing into that mainstem segment. (c) Segments shall generally be delineated according to the points at which the use, physical characteristics or water quality characteristics of a watercourse are determined to change significantly enough to require a change in use classifications and/or water quality standards 4

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As noted in paragraph 31.6(4)(c), the use or uses of surface waters are an important consideration with respect to segmentation. In Colorado there are four categories of beneficial use which are recognized. These include Aquatic Life Use, Recreational Use, Agricultural Use and Water Supply Use. A segment may be designated for any or all of these “Use Classifications”: 31.6 Waters shall be classified for the present beneficial uses of the water or the beneficial uses that may be reasonably expected in the future for which the water is suitable in its present condition or the beneficial uses for which it is to become suitable as a goal. Each assigned use is associated with a series of pollutant specific numeric standards. These pollutants may vary and are relevant to a given Classified Use. Numeric pollutant criteria are identified in sections 31.11 and 31.16 of the Basic Standards and Methodologies for Surface Water. Uses and Standards Addressed in this TMDL The Colorado Basic Standards and Methodologies for Surface Water, Regulation 31 identifies standards applicable to all surface waters statewide (WQCC 2006b). The pollutants of concern for this assessment are pH, dissolved copper, and dissolved zinc. In the case of Gamble Gulch, copper, and zinc concentrations exceed Aquatic Life Usebased standards intended to protect against short-term, acutely toxic conditions (acute) and longer-term, sub-lethal (chronic) effects. In the case of pH, the standard applies to Recreation, Aquatic Life and Water Supply uses. Gamble Gulch exceeds the pH standards for Aquatic Life and Recreation Uses. Chronic and acute standards are designed to protect against different ecological effects of pollutants (long term exposure to relatively lower pollutant concentrations vs. short term exposure to relatively higher pollutant concentrations). Where chronic standards are assigned, they were used because they represent a more conservative approach than the acute standards. Chronic standards represent the level of pollutants that protect 95 percent of the genera from chronic toxic effects of metals. By reducing metals concentrations to attain the chronic standard, the acute standard will also be attained. Per Regulation 31, chronic toxic effects include but are not limited to demonstrable abnormalities and adverse effects on survival, growth, or reproduction (WQCC 2006b). The specific numeric standards assigned to the listed stream segments are contained in Regulation 38, the Classifications and Numeric Standards for the South Platte River Basin, Laramie River Basin, Republican River Basin, and Smoky Hill River Basin (WQCC, 2006c) (Table 3). All remaining assigned numeric standards associated with Aquatic Life, Recreational, Water Supply and Agricultural Use Classifications are attained.

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Water Quality Criteria for Impaired Designated Uses WBID Impaired Designated Use Applicable Water Quality Criteria and Status pH (1) / Not Attained COSPBO04a Aquatic Life Cold 1 Dissolved Phase Cu (1) / Not Attained Dissolved Phase Zn (1) / Not Attained COSPBO04a Recreation 1a

pH (1)/ Not Attained

Applicable State or Federal Regulations: (1) Classifications and Numeric Standards for the South Platte River Basin, Laramie River Basin, Republican River Basin, and Smoky Hill River Basin (Reg 38)

Table 3. Ambient water quality criteria and status for Segment 4a, Gamble Gulch. Most of the relevant standards for the stream segment addressed in this document are Table Value Standards (TVS), which vary based on hardness. Because hardness fluctuates seasonally, standards are listed on a monthly basis using the average hardness for each month to calculate the standard. Hardness values are lowest during May and June when flows are highest (Table 4). Aquatic Life Use-based metals standards, identified as Table Value Standards or “TVS”, are typically hardness based (arsenic, mercury and selenium are exceptions). Aquatic Life Use-based TVS for metals usually are expressed as the dissolved fraction, as opposed to the total metal fraction. Again, there are exceptions, namely aluminum, iron and, again, mercury. Copper and zinc standards assigned for the protection of aquatic life are both expressed as the dissolved metal fraction and are hardness based.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Hardness mg/L 151 169 125 108 51 67 97 108 145 135 141 125

Cu-D, ug/L TVS (ch) 12.7 14.0 10.8 9.6 5.0 6.4 8.7 9.6 12.3 11.6 12.0 10.8

Cu-D, ug/L TVS (ac) 19.8 22.0 16.6 14.4 7.1 9.2 13.0 14.4 19.1 17.8 18.6 16.6

Zn-D (old) ug/L TVS (ch) 167.5 184.3 142.7 126.1 66.8 84.4 114.7 126.1 161.8 152.3 158.1 142.7

Zn-D (old) ug/L TVS (ac) 166.2 182.8 141.6 125.1 66.2 83.5 113.8 125.1 160.6 151.1 156.8 141.6

Zn-D, (new) ug/L TVS (ch) 176.6 194.4 150.3 132.7 70.0 88.3 120.6 132.7 170.6 160.5 166.6 150.3

Zn-D (new) ug/L TVS (ac) 203.7 224.2 173.4 153.1 80.7 101.9 139.1 153.1 196.8 185.2 192.1 173.4

Table 4. Average hardness and table value standards (acute and chronic) for 303(d) listed segment of Gamble Gulch. Data are from the Colorado Division of Wildlife, River Watch station 123 and Colorado Water Quality Control Division.

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Final The Colorado WQCC will convene hearings in June 2009 to consider changes to Regulation 38. Table Value Standards for zinc were revised in Regulation 31 in 2005. It is anticipated that these revisions for zinc will be adopted into Regulation 38 during the 2009 hearings, and will become effective in 2010. As such, both “old” and “new” zinc standards are presented in Table 4. Also, the nomenclature for recreation uses will be changed during this hearing from “Recreation 1a” to “Recreation E”, although the numeric standards for this use class will not change. Gamble Gulch is listed for non-attainment of the Aquatic Life Use and Recreation Usebased TVS of the pH standard. The pH standards are 6.5-9.0 s.u. for Recreation and Aquatic Life. IV.

PROBLEM IDENTIFICATION Much of the heavy metal loading throughout the South Boulder Creek basin is the result of natural geologic conditions and historic mining activities. The South Boulder Creek watershed began experiencing widespread mining activity throughout the basin beginning in the 1850’s. In June, 1859, A. D. Gambell discovered gold in Gamble Gulch in the Perigo region (miningbureau.com). The Tip Top Mine was also mined for copper, lead, zinc and silver which resulted in residual levels of elevated copper and zinc concentrations in Gamble Gulch. Currently, all of the mining features in GG are associated with abandoned mining operations. There are no permitted dischargers to Gamble Gulch. The high metals concentrations within the 303(d) listed segment of Gamble Gulch exceed the standards to protect aquatic life. Low pH values are associated with the high metals concentrations and result in exceedance of the lower pH standard for aquatic life and recreation uses. Biological studies begun in the 1980’s by the Colorado Division of Wildlife (1988), and in the 1990’s by the Colorado Water Quality Control Division (WQCD) and the U.S. Environmental Protection Agency (EPA) concluded that copper and zinc threaten trout and macroinvertebrates. From 1994 through 1999, River Watch, in conjunction with the Logan School for Creative Learning constructed a wetland in Gamble Gulch. This project was funded with Section 319 nonpoint source funds. Experiments were performed using phytoremediation to mitigate the effects of heavy metal waste from the abandoned mine site. Because of limited funding and ongoing operation and maintenance requirements, as well as liability issues, the remediation efforts were conducted as a demonstration project. As such, the phytoremediation demonstration was not continued, and copper and zinc levels, still exceed the current water quality standards. V.

WATER-QUALITY GOAL AND TARGET

The water quality goal for the 303(d) listed segment, Gamble Gulch, is attainment of the Aquatic Life Cold 1 use classification standards for copper and zinc, as well as the Aquatic Life Cold 1 and Recreation 1a use classification standards for pH.

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Final VI.

INSTREAM CONDITIONS

Hydrology The hydrograph of Middle Boulder Creek should approximate the pattern of the Gamble Gulch hydrograph, although at a much larger magnitude. Such hydrographs are typical of high mountain streams, with low flows occurring in the late fall to early spring followed by a large increase in flow, usually in May or June, due to snowmelt that tails off through the summer (Figure 2, Table 5).

Figure 2. Hydrograph of Middle Boulder Creek near Nederland, USGS gage 06725500.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Monthly Median. Flow Middle Boulder Cr. 6 5 6 20 123 223 97 41 24 16 11 7

Monthly Median. Flow Gamble Gulch. 0.4 0.3 0.4 1.3 8 14 6 3 1.5 1 0.7 0.5

Table 5. Monthly median flows (cfs) for Middle Boulder Creek and estimated monthly median flows (cfs), for Gamble Gulch (1987–2008)

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Final Flows for Middle Boulder Creek were obtained from USGS gage #6725500 near Nederland, Colorado, which operated during the years 1907 through 2007 and from Colorado Division of Water Resources (gage renamed BOCMIDCO), which operated the gage since 2007. Gamble Gulch flows were estimated using a watershed area ratio (0.0632) and applying the ratio to the data from the Middle Boulder Creek gage. Median monthly flows in Middle Boulder Creek were between eight and four hundred eighteen cubic feet per second (cfs) based on instantaneous and estimated flows in a seasonal pattern (CDPHE, 2005-2006). Estimated median monthly flows for Gamble Gulch were between 0.3 and 14 cfs. The distribution of flows for Gamble Gulch are illustrated in a “box and whiskers” plot (Figure 3). The boxes show the 25th and 75th percentiles, while the bars or whiskers show the 5th and 95th percentiles for the flow estimates. Medians are shown as markers in the boxes. The period of record from 1987 through 2008 was used.

Figure 3. Distribution of flows in Gamble Gulch. VII. ANALYSIS OF POLLUTANT SOURCES Ambient Water Quality Data Water quality data were collected by Colorado Division of Wildlife (CDOW) River Watch from 1992 to 2005 and by the Colorado Water Quality Control Division (WQCD) in 2008. River Watch data were collected from the Tip Top Mine, and from Gamble Gulch downstream from the Tip Top Mine. During the 319 wetlands phytoremediation demonstration project, water quality samples also were collected from the wetlands project cells. Because the project no longer exists, the data from the cells were not used in this evaluation.

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Final The WQCD conducted 2 synoptic sampling events during 2008. Sample sites were located upstream from the Tip Top Mine (one site), the Tip Top Mine seepage upstream from where it flows into Gamble Gulch (one site), and downstream from Gamble Gulch (two sites). The sample sites are shown on the map in Figure 1. Sampling Sites

Gamble Gulch (upstream Tip Top Mine Seepage) Tip Top Mine (Seepage) Gamble Gulch (downstream Tip Top Mine Seepage) Table Value Standards (chronic)

Hardness (mean) mg/L 16.3

pH (s.u.)

Cu-D (ug/L)

Zn-D (ug/L)

n

7.4

<5

<10

3

256 121.5

3.39 5.226.18 6.59.0

172 22

2082 444

36 126

10.6

139.3

121.5

Table 6. Gamble Gulch ambient data summary (POR = 1991-2005, 2008). Chronic Standards Ambient water quality was determined using the Colorado Division of Wildlife River Watch data (station 123) (1991-2005) and the WQCD data (2008) described above. For this analysis, the upstream site represented background conditions. Background is represented by only three sampling events conducted during 2008. The data from these sampling events showed copper and zinc concentrations were below detection, <5 ug/L and <10 ug/L, respectively. For the purpose of assigning values for natural background conditions, the one half detection level concentrations are used. Data from the Gamble Gulch sites downstream from the Tip Top Mine seepage were used to identify and characterize exceedances of the chronic water-quality standards for copper, zinc, and pH. The 85th percentile concentration for the metals was compared to the chronic standards (Table 6). The metals standards are Table Value Standards (TVS) and are expressed as hardness-based equations. The standards were calculated using the mean hardness value of 121.5 mg/L from the available data for the period of record. Gamble Gulch water quality exceeded the lower standard for pH in addition to exceeding the chronic standards for both copper and zinc. The water quality standards for pH includes lower and upper standards and are 6.5 s.u. and 9.0 s.u., respectively. These standards apply to aquatic life and recreational uses. The water supply use pH standards are 5.0 s.u. and 9.0 s.u. For assessments, the 15th percentile of the ambient data is compared to the lower standard and the 85th percentile of the ambient data is compared to the upper standard. Gamble Gulch exceeds the lower pH standard for recreation and aquatic life uses. The data also were evaluated on a monthly basis. For this evaluation, monthly mean hardness values were used to calculate the TVS. Attainment of chronic Aquatic Life Use-based standards is based upon the 85th percentile of the ranked data. Percentile values are calculated by ranking individual data points in order of magnitude. Hardness10

Final based metal standards are evaluated by comparing the 85th percentile value against the assigned hardness-based standard (typically calculated using the mean hardness). Current available copper and zinc data are summarized in Table 7 below. Hardness values varied seasonally with flow, with the lowest values found in May and June while the highest values are in January and February. Data from Gamble Gulch from 1992-2008 demonstrate that Gamble Gulch exceeded the chronic water quality standard for copper for all months except January (Table 7, Figure 4). The highest copper values are observed in the months of May, July and September at 29.3 34.0 and 31.2 μg/L, respectively. Levels of copper toxicity are exacerbated by the fact that in May and June, hardness values are at their lowest. Zinc values exceed the chronic standard for the entire year with zinc values exceeding 400ug/L from July through December (Table 7, Figure 4). The assessment for pH evaluated on a monthly basis indicates exceedance of the lower pH standard for water supply uses. As noted above, the standard is 5.0s.u., and the data for July are 4.99 s.u. (Table 7). This indicates exceedance of the water supply use. However, the assessment for identifying impairments is based on the 15th percentile of all the data. It is assumed that efforts to attain standard for the listed impairments will address this water supply use exceedance.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Hardness 151 169 125 108 51 67 97 108 145 135 141 125

pH Std. 6.5-9.0 6.5-9.0 6.5-9.0 6.5-9.0 6.5-9.0 6.5-9.0 6.5-9.0 6.5-9.0 6.5-9.0 6.5-9.0 6.5-9.0 6.5-9.0

pH 15th percentile (n=118) 5.31 5.22 5.36 5.46 5.24 5.32 4.99 5.05 5.10 5.53 5.41 5.48

Gamble Gulch pH Cu-D, 85th TVS percentile (ch) 5.83 5.83 6.05 6.50 6.75 6.09 5.98 6.01 6.26 5.93 5.92 6.19

12.7 14.0 10.8 9.6 5.0 6.4 8.7 9.6 12.3 11.6 12.0 10.8

Cu-D (n=126) 8.8 14.7 11.0 11.4 29.3 20.2 34.0 19.1 31.2 23.8 18.1 19.7

Zn-D, TVS (ch) 176.6 194.4 150.3 132.7 70.0 88.3 120.6 132.7 170.6 160.5 166.6 150.3

Zn-D (n=128) 360.0 374.2 336.2 281.9 213.3 308.4 422.1 425.9 501.5 480.3 452.1 451.2

Table 7. Ambient water quality data for Gamble Gulch. Ambient concentrations are calculated as 85th %.

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Figure 4. Ambient eighty-fifth percentile monthly copper and zinc concentrations for Gamble Gulch. Load Duration Curves Load duration curves are graphical analytical tools that illustrate the relationships between stream flow and water quality. Flow is an important factor affecting the loading and concentration of metals. Load duration curves are used to characterize water quality data at different flow regimes. A load duration curve consists of a curve that represents the water quality standard of interest and is developed by multiplying stream flow with the numeric water quality target and a conversion factor for the pollutant of concern. This curve, the load duration curve, plotted as a continuous line, represents the loading capacity or allowable load for the water body. Ambient water quality data, taken with a flow measurement associated with the time of sampling, for example, daily mean flow, is used to compute an instantaneous load. By plotting the instantaneous loads with the load duration curve, characteristics of water quality impairment can be described. 12

Final Instantaneous loads that plot above the curve indicate exceedance of the water quality criterion, while loads that plot below the load duration curve illustrate compliance. The pattern of impairment is examined to see if impairments occur across all flow conditions or under certain flow regimes. For example, impairments observed in the low flow zone typically indicate the influence of point sources, while impairments toward the left side of the curve typically reflect nonpoint source contributions. Load duration curves for Gamble Gulch were constructed to provide further illustration comparing loads to the standards across all hydrologic conditions (Figure 5). Both copper and zinc exceed standards under all flow conditions, which suggests pollutant contributions from groundwater sources, point sources, and additional nonpoint sources from mining features.

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Figure 5. Load duration curves for dissolved copper and dissolved zinc. Acute Standards Acute standards are evaluated by comparison of single sample values to standard. The standard is calculated for each sampling event based upon the discrete, sample specific hardness. Data indicate non-attainment of an acute standard if the standard is exceeded more frequently than once in three years. Attainment of the acute standards for copper and zinc was assessed for the data from sites downstream from the Tip Top Mine seepage. For this assessment, only samples with paired hardness and copper or paired hardness and zinc were used. Acute standards for both metals were exceeded. For copper, acute exceedances were observed for 40 out of 123 samples (27%). Figure 6 illustrates the pattern of exceedances by month for dissolved copper. Acute standards for dissolved copper were attained during January through March, but are exceeded for the remaining months of the year. For zinc, 119 of 122 samples exceeded the acute standards (81.5%). Figure 7 shows acute exceedances for all months for dissolved zinc.

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Figure 6. Exceedances of acute standards for dissolved copper.

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Figure 7. Exceedances of acute standards for dissolved zinc.

VIII. TMDL Allocation A TMDL is comprised of the Load Allocation (LA), which is that portion of the pollutant load attributed to natural background and/or the nonpoint sources, the Waste Load Allocation (WLA), which is that portion of the pollutant load associated with point source discharges, and a Margin of Safety (MOS). The TMDL may be expressed as the sum of the LA, WLA and MOS. TMDL = WLA + LA + MOS TMDL = Sum of Waste Load Allocations + Sum of Load Allocations + Margin of Safety Waste Load Allocations “(WLA)” There are no identified permitted point sources to this segment. The only source found was the Tip Top Mine seepage located near the upper third of this tributary; however there is no CPDES permit for the mine. Limited data for flows from this source were available, although adequate water quality data were available. Discharge from the mine will be treated as a non-permitted discharge in this TMDL and will be given a waste load allocation. Load Allocations “(LA)” Any remaining sources are considered to be non-point sources and are accountable to load allocations.

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Final Margin of Safety “(MOS)” According to the Federal Clean Water Act, TMDLs require a margin of safety (MOS) component that accounts for the uncertainty about the relationship between the pollutant loads and the receiving waterbody. The margin of safety may be explicit (a separate value in the TMDL) or implicit (included in factors determining the TMDL). In the case of the Gamble Gulch TMDL, a 10% margin of safety was used. As a result, proposed reductions also address exceedances of the acute standards assigned to the listed segment. The TMDL is calculated using month median flows (estimated from USGS gage #06725500 as described in section VI above), multiplied by the existing stream standard and a conversion factor (0.0054) to approximate a load in pounds/day. Eighty-fifth percentile concentrations are calculated on a monthly basis and multiplied by monthly median flows and a conversion factor (0.0054) to estimate a daily load in pounds/day. This load is reduced by 10% to reflect the margin of safety. The resulting load is allocated between background nonpoint source for the Load Allocation and the discrete and diffuse sources at the Tip Top Mine site for the Waste Load Allocation. The TMDL allocations (LA and WLA) are determined by calculating the proportion from background and attributing the remainder to mining influences. Background is based on the concentrations from the upstream site. The water quality at this site was below detection levels for copper and zinc. Although the Division’s assessment methodology assigns a value of zero to non-detects, for the purpose of determining load allocations, one half detection is assigned for the background concentration. The assigned background concentration for copper is 2.5 ug/L. For zinc, the assigned background concentration is 5 ug/L. The proportion of background was calculated by dividing the background concentration by the annual average of monthly concentrations of the ambient (observed) concentration for the downstream site. The background copper concentration was twelve percent (12 %) of the observed concentration. The remaining eighty-eight percent is attributed to mining influence. The background zinc concentration was one percent (1 %) of the observed concentration. The remaining ninety-nine percent is attributed to mining influence. The resulting percentage is multiplied by the calculated TMDL to determine the Load Allocation. The Waste Load Allocation is calculated as the difference between the TMDL and the LA. Tables 8 and 9 present the TMDL, LA, and WLA by month for copper and zinc, respectively. Note that for copper, the reduction for January is a negative value. This indicates the standard is being attained, and no reductions are necessary in January. The zinc standard for this segment will change during 2009. The revised standard will be slightly higher than the current standard. Therefore, the TMDL based on the revised standard is slightly higher, as well, while the associated reductions are slightly lower.

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Final

Table 8. Cu TMDL and Load Reduction by month (includes 10% MOS) Segment: COSPBO04a Gamble Gulch (n= 126 ) Cu-D TMDL Reduction Reduction TMDL LA Observed Load Load Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

(lbs/D) 0.018 0.028 0.024 0.082 0.893 1.596 1.137 0.270 0.285 0.140 0.069 0.048

(lbs/D) 0.023 0.022 0.020 0.059 0.190 0.435 0.259 0.121 0.091 0.057 0.041 0.024

(lbs/D) -0.0048 0.0057 0.0033 0.0232 0.7035 1.1610 0.8782 0.1490 0.1941 0.0827 0.0287 0.0244

% -27% 21% 14% 28% 79% 73% 77% 55% 68% 59% 41% 51%

(lbs/D) 0.003 0.003 0.003 0.007 0.023 0.054 0.032 0.015 0.011 0.007 0.005 0.003

Table 9. Zn TMDL and Load Reduction by month (includes 10% MOS) Segment: COSPBO04a Gamble Gulch (n=128 ) Zn-D TMDL Reduction Reduction TMDL LA Observed Load Load Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

(lbs/D) 0.738 0.681 0.703 1.961 9.787 23.912 14.240 6.082 4.251 2.687 1.709 1.103

(lbs/D) 0.30 0.29 0.27 0.78 2.51 5.76 3.42 1.59 1.19 0.75 0.53 0.31

(lbs/D) 0.43 0.39 0.44 1.19 7.27 18.16 10.82 4.49 3.06 1.94 1.17 0.79

% 59% 58% 62% 60% 74% 76% 76% 74% 72% 72% 69% 72%

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(lbs/D) 0.004 0.004 0.003 0.010 0.032 0.073 0.044 0.020 0.015 0.010 0.007 0.004

TMDL WLA

(lbs/D) 0.020 0.019 0.018 0.052 0.167 0.383 0.228 0.106 0.080 0.050 0.036 0.021

TMDL WLA

(lbs/D) 0.300 0.285 0.264 0.765 2.482 5.681 3.376 1.569 1.179 0.740 0.528 0.307

Final

Table 10 presents the TMDL based on the revised standard. Table 10. Zn TMDL and Load Reduction by month (includes 10% MOS) Segment: COSPBO04a Gamble Gulch (n=128 ) Zn-D TMDL Reduction Reduction TMDL LA Observed Load Load Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

(lbs/D) 0.74 0.68 0.70 1.96 9.79 23.91 14.24 6.08 4.25 2.69 1.71 1.10

(lbs/D) 0.32 0.30 0.28 0.82 2.64 6.04 3.60 1.67 1.26 0.79 0.56 0.33

(lbs/D) 0.42 0.38 0.42 1.15 7.15 17.87 10.64 4.41 2.99 1.90 1.15 0.78

% 57% 55% 60% 58% 73% 75% 75% 72% 70% 71% 67% 70%

(lbs/D) 0.004 0.004 0.004 0.010 0.034 0.077 0.046 0.021 0.016 0.010 0.007 0.004

TMDL WLA

(lbs/D) 0.316 0.301 0.278 0.806 2.603 5.967 3.552 1.652 1.243 0.780 0.556 0.324

Acute Standards Attainment of acute standards was evaluated by applying the monthly reduction percentages identified in the tables above to individual samples. These reductions resulted in attainment of the acute standards for copper in 121 of 123 samples (2 exceedances). For zinc, the reductions resulted in attainment of the acute standards in 117 of 122 samples (5 exceedances). Although acute exceedances were estimated for both copper and zinc, these exceedances were for samples collected prior to 2000. In the Division’s assessments for attainment of standards, assessments are based on the most recent 5 years of data. In the Gamble Gulch data from 2000-2008, no acute exceedances for copper or zinc would be observed with the TMDL reductions. Based on this rationale, acute standards for copper and zinc would be attained through the above TMDLs. pH TMDL Gamble Gulch is impaired due to pH exceedances. Both the fifteenth and eighty-fifth percentiles of the observed data are below the lower pH standard of 6.5. It is expected that remediation for copper and zinc, by addressing sources of acid mine drainage, will result in attainment of the pH standard. Post-remediation monitoring can verify this assumption.

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Final IX.

RESTORATION PLANNING AND IMPLEMENTATION PROCESS

The monthly percentages of loading reduction necessary to meet TVS standards for copper and zinc on Gamble Gulch are listed in Table 8 and 9. The major source contributing to the elevated level of metals in Gamble Gulch is the Tip Top Mine and non-permitted discharge from the Tip Top mine property. A substantial reduction of metals from this non-permitted point source is necessary to attain current TVS standards in Gamble Gulch. There is no known copper and zinc remediation planned for Gamble Gulch. Previous Water Quality Improvements in the Watershed In 1994 through 1999, River Watch in conjunction with the Logan School for Creative Learning received non-point source funding to construct a wetland in Gamble Gulch. Experiments were performed using phytoremediation to mitigate the effects of heavy metal waste from the abandoned mine site. The mitigation was not ongoing, therefore, copper and zinc levels do, however, still exceed the current water quality standards. Monitoring Additional monitoring of Gamble Gulch is not planned at this time. If remediation for copper and zinc is implemented, monitoring of Gamble Gulch should be required in order to ensure that the TMDL is adequately protective of the segment. Additional water quality and flow monitoring of the drainage from the Tip Top Mine as well as from Gamble Gulch upstream and downstream of the mine would be included for comprehensive monitoring for any remediation efforts. Conclusion The goal of this TMDL is the attainment of the TVS for copper, zinc, and pH within the Gamble Gulch portion of Segment 4a of the South Boulder Creek. Substantial loading reductions of both metals are necessary to attain the TMDL for each metal. Improvements in pH would result from metals remediation. The recommended loading reductions should result in attainment of both chronic and acute water quality standards. X.

PUBLIC INVOLVEMENT

This segment was included on Colorado’s 303(d) list of impaired segments in 2006. The development of the 303(d) list is a public process involving solicitation from the public of candidate waterbodies, formation of a technical review committee comprised of representatives of both the public and private sector, and a public hearing before the Colorado Water Quality Control Commission. Public notice is provided concerning both the solicitation of impaired waterbodies and the public hearing.

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Final The TMDL itself is the subject of an independent public process. This TMDL report was made available for public review and comment during a 30 day public notice period in June, 2009. The EPA provided minimal comments on the draft TMDL. The EPA comments included request for corrections in the standards formulas in the summary table, request for raw data used in the TMDL analysis, and identification of public notice comments. The WQCD received no additional comments during the public notice period. References Lehnertz, C.S., 1988. Biological Assessment of Gamble Gulch. Colorado Division of Wildlife. Miningbureau.com (How to reference this?) Thomann, R.V., and J.A. Mueller. 1987. Principles of Surface Water Quality Modeling and Control. Harper & Row, New York, NY. USGS. 2000. Comprehensive Water Quality of the Boulder Creek Watershed, Colorado, During High-Flow and Low-Flow Conditions. Murphy, S. F., P.L. Verplanck, and L.B. Barber, ed. Water-Resources Investigations Report 03-4045. WQCC 2006a. Colorado Department of Public Health and Environment, Water Quality Control Commission, 2006, 303(d) List of Impaired Waters, 2006. WQCC 2006b. Colorado Department of Public Health and Environment, Water Quality Control Commission, The Basic Standards and Methodologies for Surface Water, Regulation No. 31. Effective December 31, 2005. WQCC 2005. Colorado Department of Public Health and Environment, Water Quality Control Commission, Classification and Numeric Standards South Platte River Basin, Laramie River Basin, Republican River Basin, Smokey Hill River Basin, Regulation 38, amended effective January 20, 2005.

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