UNITED STATES ENVIRONMENTAL PROTECTION AGENCY REGION 8 1595 Wynkoop Street DENVER, CO 80202-1129 Phone 800-227-8917 httpYhvww.epa.govlregion08
Ref: 8EPR-EP
Mr. Steve Gunderson Director Water Quality Control Division Colorado Department of Public Health and Environment 43 0Q Cherry Creek Drive South Denver, Colorado 80246-1 530
Re: TMDL Approvals Gamble Gulch COSPBOOda cadmium and zinc
Deaf Mr. Gunderson: We have completed our review of the total maximum daity loads (TMDLs) as submitted by your ofice on July 6 2 0 10 for the waterbody listed in the enclosure to this letter. In accordance with the Clean Water Act (33 U.S.C. 125 1 et. seq.), we approve all aspects of TMDLs as developed for certain pollutants in water quality limited waterbodies as described in Section 303(d)(l). Based on our review, we feel the separate TMDL elements for the pollutants listed in the enclosed table are adequately addressed, taking into consideration seasonal variation and a margin o f safety. Thank you for submitting these TMDLs for our review and approval. I f you have any questions, the most knowledgeable person on my staff is Julie Kinsey and she may be reached at (303) 3 12-7065.
Carol L. Campbell Assistant Regional Administrator Ofice of Ecosystems Protection md Remediation
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Printed on Recycled Paper
Final June 2010
TOTAL MAXIMUM DAILY LOAD ASSESSMENT GAMBLE GULCH COSPBO04A Cadmium and Zinc GILPIN COUNTY, COLORADO June 2010
TMDL Summary Waterbody Description / WBID
Mainstem of South Boulder Creek, including all tributaries, lakes, reservoirs, and wetlands, from the source to the outlet with Gross Reservoir, COSPBO04a
Dissolved cadmium, Dissolved zinc, Pollutants Addressed Relevant Portion of Gamble Gulch Segment (as applicable) Use Classifications / Aquatic Life Cold 1, Recreation 1a, Water Supply, Agriculture; Designation Water Quality Target Segment Chronic Acute 4a
TMDL Goal
Cd-D
Chronic=(1.101672[ln(hardness) x (0.041838)]) x e(0.7998[ln(hardness)]-4.4451
Zn-D (2010)
TVS=0.986e0.8525[ln(hardness)]+0.9109) TVS=0.978e0.8525[ln(hardness)]+1.0617)
TVS(Trout)=(1.136672[ln(hardness)x (0.041838)] )x e0.9151[ln(hardness)]-3.6236
Attainment of Aquatic Life use classification standards for Cd.
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EXECUTIVE SUMMARY The Gamble Gulch portion of Boulder Creek Segment 4a (COSPBO04a)had 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. A TMDL to address those parameters was approved by EPA April, 2009. The segment re-appeared on the 2010 303(d) List, this time due to non-attainment of the assigned Aquatic Life Use-based zinc and cadmium standards. This TMDL document addresses non-attainment of the revised zinc and cadmium standards.
I. BACKGROUND A TMDL report for Boulder Creek Segment 4a, Gamble Gulch, for copper, zinc, and pH was submitted to and approved by EPA in 2009. At the time the original report was prepared, it was known that revised zinc standards would become effective in January 2010. Although only the TMDL for the existing zinc standard was submitted for approval, the original report included the TMDL for the revised zinc standard. In addition to a revised zinc standard, a revised cadmium standard also became effective in January 2010. Gamble Gulch does not attain the revised cadmium and zinc standards. Segment COSPBO04a, Gamble Gulch re-appeared on the 2010 303(d) List, this time due to non-attainment of the assigned Aquatic Life Use-based zinc and cadmium standards (Table 1). This document adds the dissolved cadmium TMDL and dissolved zinc to the previously completed zinc, copper and pH TMDLs. At this time, the Division will submit for approval, TMDLs for revised dissolved zinc and dissolved cadmium standards.
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 Cd, Zn
Table 1. Segment within the Boulder Creek watershed that appears on the 2010 303(d) list of impaired water bodies.
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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 2010 303(d) list for exceeding the water quality standards for dissolved cadmium 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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Date (Cycle Year) of Current Approved 303(d) list: 2008, 2010
WBID
Segment Description
Mainstem of South Boulder Creek, including all tributaries, COSPBO04a lakes, reservoirs, and wetlands, from the source to the outlet with Gross Reservoir.
Designated Uses & Impairment Status Aquatic Life Cold 1: Impaired Recreation 1a: Not 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).
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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.
II.
WATER QUALITY STANDARDS
Standards Framework
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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 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 dissolved cadmium and dissolved zinc. In the case of Gamble Gulch, cadmium and zinc concentrations exceed Aquatic Life Use-based standards intended to protect against short-term, acutely toxic conditions (acute) and longer-term, sub-lethal (chronic) effects. 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
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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.
Water Quality Criteria for Impaired Designated Uses
WBID
Impaired Designated Use
Applicable Water Quality Criteria and Status pH (1) / Not Attained (addressed in previous TMDL) Dissolved Phase Cu (1) / Not Attained (addressed in previous TMDL)
COSPBO04a
Aquatic Life Cold 1
Dissolved Phase Zn (1) / Not Attained (addressed in previous TMDL) Dissolved Phase Zn (1), (revised Zn Std.) / Not Attained Dissolved Phase Cd (1), (revised Cd Std.) / Not Attained
COSPBO04a Recreation 1a
pH (1)/ Not Attained(addressed in previous TMDL)
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
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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 Usebased 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. Cadmium and zinc standards assigned for the protection of aquatic life are both expressed as the dissolved metal fraction and are hardness based.
Zn-D, ug/L
Zn-D, ug/L
mg/L
TVS (ch)
Cd-D, ug/L TVS (actrout)
TVS (ch)
TVS (ac)
Jan
151
0.58
2.44
176.6
203.7
Feb
169
0.63
2.69
194.4
224.2
Mar
125
0.50
2.07
150.3
173.4
Apr
108
0.45
1.82
132.7
153.1
May
51
0.26
0.95
70.0
80.7
Jun
67
0.31
1.20
88.3
101.9
Jul
97
0.41
1.65
120.6
139.1
Aug
108
0.45
1.82
132.7
153.1
Sep
145
0.56
2.35
170.6
196.8
Oct
135
0.53
2.21
160.5
185.2
Nov
141
0.55
2.30
166.6
192.1
Dec
125
0.50
2.07
150.3
173.4
Cd-D, ug/L Hardness
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. III. PROBLEM IDENTIFICATION
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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. 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 the high metals concentrations 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. Cadmium and zinc levels exceed the current water quality standards. IV. 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 cadmium and zinc. V. 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).
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Figure 2. Hydrograph of Middle Boulder Creek near Nederland, USGS gage 06725500.
Monthly Median. Flow Middle Boulder Cr.
Monthly Median. Flow Gamble Gulch.
Jan
6
0.4
Feb
5
0.3
Mar
6
0.4
Apr
20
1.3
May
123
8
Jun
223
14
Jul
97
6
Aug
41
3
Sep
24
1.5
Oct
16
1
Nov
11
0.7
Dec
7
0.5
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Table 5. Monthly median flows (cfs) for Middle Boulder Creek and estimated monthly median flows (cfs), for Gamble Gulch (1987–2008) 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. VI. ANALYSIS OF POLLUTANT SOURCES Ambient Water Quality Data
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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. 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. Ambient data from these monitoring sites are summarized in Table 6. Data from monitored sites downstream from the Tip Top Mine seepage are used to characterize exceedances of the cadmium standards. Sampling Sites
Hardness (mean)
Cd-D
Zn-D
(ug/L)
(ug/L) <10 (n=3)
mg/L Gamble Gulch (upstream Tip Top Mine Seepage)
16.3
<1 (n=3)
Tip Top Mine (Seepage)
256
7.64 (n=37)
Gamble Gulch (downstream Tip Top Mine Seepage)
121.5
Table Value Standards (chronic)
121.5
2082 (n=36) 1.58 444 (n=125) (n=128) 0.49
146.75
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 cadmium and zinc concentrations were below detection, <1 ug/L and <10 ug/L, respectively. For the purpose of assigning values for natural background conditions, for zinc, one half detection level concentrations are used. This was the approach used for zinc in the original TMDL.
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To assign values for natural background conditions for cadmium, less than detection is assigned 0. This is the approach typically used in State of Colorado assessments. It is applied here because the cadmium standard is less than 1. Also, there is uncertainty in the reported value of <1.
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 cadmium and zinc. 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. 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. Hardness-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 cadmium 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 cadmium for all months. The highest cadmium values are observed in the months of July, October and December at 1.63, 2.01 and 1.82 g/L, respectively. Zinc values exceed the chronic standard for the entire year with zinc values exceeding 400ug/L from July through December (Table 7, Figure 4).
Hardness Jan Feb Mar Apr May Jun Jul Aug
151 169 125 108 51 67 97 108
CdD, 0.58 0.63 0.50 0.45 0.26 0.31 0.41 0.45
Gamble Gulch Cd-D Zn-D, (ug/L) TVS 1.32 176.6 1.46 194.4 1.01 150.3 1.32 132.7 1.17 70.0 1.38 88.3 1.63 120.6 1.53 132.7
Zn-D (ug/L) 360.0 374.2 336.2 281.9 213.3 308.4 422.1 425.9
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Sep 145 0.56 1.46 170.6 501.5 Oct 135 0.53 2.01 160.5 480.3 Nov 141 0.55 1.42 166.6 452.1 Dec 125 0.50 1.82 150.3 451.2 Table 7. Ambient water quality data for Gamble Gulch. Ambient concentrations are calculated as 85th %.
Figure 4. Ambient eighty-fifth percentile monthly cadmium and zinc concentrations for Gamble Gulch.
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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. 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). The revised cadmium standard is more stringent (lower) than the old cadmium standard. The load duration curve for cadmium illustrates that the standard is exceeded under all flow regimes. For zinc, the revised standard is less stringent (higher) than the old zinc standard, but the standard still is exceeded under all flow regimes.
High Flows
Moist Conditions
Mid‐range Conditions
Dry Conditions
Low Flows
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High Flow
Moist Conditions
Mid‐range Conditions
Dry Conditions
Low Flows
Figure 5. Load duration curves for dissolved cadmium and dissolved zinc.
Acute Standards Acute standards are evaluated by comparison of single sample values to the standard. The standard is calculated for each sampling event based on the discrete, sample-specific hardness. Non-attainment of the acute standard is determined if the standard is exceeded more frequently than once in three years. For Gamble Gulch, data from sites downstream from the Tip Top Mine seepage were assessed for attainment of the acute standards for dissolved cadmium and dissolved zinc. Only samples with paired hardness and cadmium or paired hardness and zinc were used. For Gamble Gulch, the acute cadmium standard is the acute trout standard. For cadmium acute trout , 6 of 122 samples exceeded the acute standard (n=122). As stated above, non-attainment is determined if exceedances occur more frequently than once in three years. Acute exceedances were observed once in 1992, twice in 1998, once in 1999, and twice in 2003. No exceedances have been observed since 2003. Therefore, in the case of cadmium, Gamble Gulch attains the acute standard. Figure 6 illustrates the pattern of exceedances by month for dissolved cadmium. However, for zinc, 118 of 122 samples exceeded the acute standards, and Gamble Gulch is in non-attainment of the acute zinc standard. Figure 7 shows acute exceedances for all months for dissolved zinc.
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Figure 6. Exceedances of acute standards for dissolved cadmium.
Figure 7. Exceedances of acute standards for dissolved zinc.
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.
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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. 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 monthly 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 cadmium. The Division’s assessment methodology assigns a value of zero to non-detects. Therefore, the background concentration of dissolved cadmium is assigned the value of 0, and the load allocation is 0. The observed concentrations of cadmium at the downstream site are attributed to mining influence, and the entire cadmium TMDL is allocated to the WLA. Although the Division’s assessment methodology assigns a value of zero to non-detects, for the
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purpose of determining load allocations for zinc, one half detection is assigned for the background concentration. 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 zinc concentration was one percent (1 %) of the observed concentration. The remaining ninety-nine percent is attributed to mining influence. Tables 14and 15 present the TMDL, LA, and WLA by month for cadmium and zinc, respectively. Please note that the new zinc TMDL was in the original document in table 10 of that document.
Mont Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Table 14. Cd TMDL and Load Reduction by month (includes 10% MOS) Segment: COSPBO04a Gamble Gulch (n=128 ) Cd-D TMD MO TMDL Reducti Reducti TMDL TMDL Observe L S Load on on LA WLA d Load Load (w/10% MOS) (lbs/D) (lbs/ (lbs/ (lbs/D) (lbs/D) % (lbs/D) (lbs/D) 0.0028 0.001 0.00 0.0010 0.0018 63% 0.000 0.0010 0.0027 0.001 0.00 0.0010 0.0017 63% 0.000 0.0010 0.0021 0.001 0.00 0.0009 0.0012 56% 0.000 0.0009 0.0109 0.003 0.00 0.0028 0.0082 75% 0.000 0.0028 0.0397 0.010 0.00 0.0096 0.0301 76% 0.000 0.0096 0.1087 0.023 0.00 0.0215 0.0872 80% 0.000 0.0215 0.0540 0.013 0.00 0.0123 0.0417 77% 0.000 0.0123 0.0215 0.006 0.00 0.0057 0.0158 74% 0.000 0.0057 0.0128 0.004 0.00 0.0041 0.0086 68% 0.000 0.0041 0.0116 0.002 0.00 0.0026 0.0090 77% 0.000 0.0026 0.0060 0.002 0.00 0.0019 0.0042 69% 0.000 0.0019 0.0046 0.001 0.00 0.0011 0.0035 76% 0.000 0.0011
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Mont Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Table 15. Zn TMDL and Load Reduction by month (includes 10% MOS) Segment: COSPBO04a Gamble Gulch (n=128 ) Zn-D TMD MO TMDL Reducti Reducti TMDL TMDL Observe L S Load on on LA WLA d Load Load (w/10% MOS) (lbs/D) (lbs/ (lbs/ (lbs/D) (lbs/D) % (lbs/D) (lbs/D) 0.738 0.36 0.04 0.32 0.42 57% 0.004 0.316 0.681 0.34 0.03 0.30 0.38 55% 0.004 0.301 0.703 0.31 0.03 0.28 0.42 60% 0.004 0.278 1.961 0.91 0.09 0.82 1.15 58% 0.010 0.806 9.787 2.93 0.29 2.64 7.15 73% 0.034 2.603 23.912 6.72 0.67 6.04 17.87 75% 0.077 5.967 14.240 4.00 0.40 3.60 10.64 75% 0.046 3.552 6.082 1.86 0.19 1.67 4.41 72% 0.021 1.652 4.251 1.40 0.14 1.26 2.99 70% 0.016 1.243 2.687 0.88 0.09 0.79 1.90 71% 0.010 0.780 1.709 0.63 0.06 0.56 1.15 67% 0.007 0.556 1.103 0.36 0.04 0.33 0.78 70% 0.004 0.324
. 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 cadmium in 122 of 122 samples. For zinc, the reductions resulted in attainment of the acute standards in 120 of 122 samples (2 exceedances). Although acute exceedances were estimated for zinc, these exceedances were for samples collected in 1991 and 1992. 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 cadmium or zinc would be observed with the TMDL reductions. Based on this rationale, acute standards for cadmium and zinc would be attained through the above TMDLs.
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VII.
RESTORATION PLANNING AND IMPLEMENTATION PROCESS
The monthly percentages of loading reduction necessary to meet TVS standards for cadmium and zinc on Gamble Gulch are listed in Table 14 and 15. 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 cadmium and zinc remediation planned for Gamble Gulch at this time.
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, cadmium 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 cadmium 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 cadmium and zinc 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. The recommended loading reductions should result in attainment of both chronic and acute water quality standards.
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VIII. PUBLIC INVOLVEMENT This segment was included on Colorado’s 303(d) list of impaired segments in 2006 through 2010. 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. 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 May, 2010. No comments were received during the public notice period.
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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. WQCC 2010. 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 1, 2010.
