HYDRO29057

• Permit No: M1988112
• IBM Index Class Name: Hydrology
• Doc Name: TABLE OF CONTENTS
• Media Type: D
• Archive: No

Appendu D, Comments from Rocky Mountain Consultants, /nc., Hydrokinetics, /nc., & Hydrodynamics,/nc. (Cont.) Page 12
Comment:
5. Section ll/.B.1. Dnze 3
The use of the 85'" percentile data for RS-5 is inconsistent with the use of the Sd" percentile data jar RS-/. The use of a lower
percentile jar the upstream loading allows for a larger loading jrorn the discharge point(s). 77re same percentile loading must
be considered for both the upstream and downstream points.
Stations RS-2 and RS-5 are located opprorimately 3,000 jeer and 9,000jeet downstrean of the West Pit, respectivety. A
comparison ojloading from stations RS-2 and RS-5 must be provided to demons[rate that station RS-5 is the appropriate location
to determine baseline loadings. We believe drat geochemical reactions and additional loadings in the more t/ran one mile
between Stations RS-2 and RS-5 make RS-2 a more appropriate selection jar this analysis.
As discussed in Cornrnent 44, x~e also reconunend that the metals data be tested jar seasonality to determine if the use of
seasonal concentration values is appropriate, rather than using the 50" or 85`" percentile values jar all Jlaw regimes.
Response:
According to the existing Division policy, the upstream background (MI) value is determined by the 50" percentile, and the
ambient based values for water quality standards (M3) are determined Jrom the 85'" percentile. There is no change, since this
same procedure is used for other CDPS permits as well as jar establishing ambient standards for stream segments.
The Division believes that the range ojinJluences as recorded in water quality from RS-5 provides a beret measure ojthe range
of natural hydrologic and geochemical processes influencing water quality in Ure reach ojthe Rim Seco that is the receiving
water jar the water treatment facility. Also refer to previous discussions (page 24 ojrhe ratowle and page 6 oJAppendiz D).
The issue oJseasonaliry is implicitly addressed by the structure of the loading calculation. 77rat is, theJlows used to calculate
loading correspond to the low-/low period of the year when the concentrations ojconstituents at the upstream and downstream
locations would be the highest annually. Greater natural loading would probably occur during the spring snowrnelt when the
flows are greatest and concentrations are typically ar annual [ow values. Therefore, the permissible constituent loading
calculated xdll be conservatively low. /Jdetermined, seasonal limitations for many parameters would not be expected to be
significantly different jrorn the annual determined limits. Also, there is not enough existing da(a to adequately determine
seasonal limits.
Comment:
6. Section III.B.2. Dare 3
The permit provides no information concerning the calculation of the !E3 and 30E3 low flow values. Specifically, what station
were these values derived jrorn? How manyJlow values x~ere used in the calculations? M7rat is the period ojrecord (e.g., does
it include a mixture of "wet ", "drv ", and "normal" years)? Absen[ this documentation, the app[icabiliry of these flow values as
the IE3 and 30E3 lowJlaw values can not be justified.
Response:
The stream low flows were established by the Division in accordance with standard procedures used for determining low flow
values for all COPS permits. /t is not appropriate to include all these details in the rationale, since muc/r of this injormaton is
included in the permit file. Any other questions regarding the deternination of stream law flow values would need to be referred
to the Assessment Unit ojthe Division.
Comment:
7. Section /1/.B.3. Daze 3
The Shalom Ranch Headgate is located less than S miles downstream of the facility.
Response:
This parlor ojthe rationale is typically used to indicate ijthere are any public surface drinking rater intakes x~i[/rin a 5 mile
radius downstream of a discharge. Additional discussions relating to the other downstream/downgradient uses have been
indicated in this section.
The Shalom Ranc/r Headgate divers water from the Rito Seco for agricultural use. A comparison of the historical water quality
data record for the Shalom Ranch Headgate and the surface water station RS-S (data that are in the CDPHE files) show that the
wa[er quality at the S/ralom Ranch headgate generally mirror the quality of water as measured at RS-5.