FLOOD04954

• County: Costilla
• Community: San Luis
• Basin: Rio Grande
• Title: Subsurface Drainage Feasibilty Study
• Date: 9/1/1996
• Prepared For: Costilla County
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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V. GROUNDWATER INVESTIGATION

On April 25 and 26, 1988 under the supervision of WRC Engineering, Custom Auger
Drilling Company of Denver drilled 13 monitoring wells at various locations throughout
the Town of San Luis. These wells were installed to gather base data for analyzing the
community's groundwater problems. The locations of the monitoring wells is shown on
figure 3.

The wells were drilled to depths ranging from 10 feet to 27 feet. Soil logs and depths to
groundwater at the time of drilling are shown on figure 4. At the time of drilling,
groundwater was present in all but three of the wells. Water was encountered at depths
ranging from 3 feet to 17 feet below the ground surface. The maximum groundwater
elevations in monitoring wells 7, 8, and 9 (during the period of May 1988 through August
1988) are shown in table 1. In the southern part of town, the presence of organic clay at
and near the surface indicates that swampy conditions have likely existed there for a long
period of time. The number of artesian wells south of Gaspar Street may be contributing
to the swampy conditions.

Properties of the groundwater aquifer were determined through sieve analyses on soil
samples from 4 of the monitoring wells after pumping tests failed. WRC Engineering
determined that the groundwater interception rate required for the subdrain would be 0.73
cfs based on a length of 1200 feet and lowering the water table by a maximum of 4 feet.
Additional information regarding the groundwater characteristics, soils information,
monitoring wells, and pumping tests can be found in the report entitled "Master Drainage
Study, Town of San Luis, Colorado" (WRC Engineering, 1988).

For the purposes of this current feasibility study, the results of the previous groundwater
investigation will be used in the development of preliminary project designs. The lack of
funding available at the time of this study precluded the possibility of performing
additional groundwater investigations.

Table 1
1988 Maximum Groundwater Elevations

Well # Ground Elev. (ft) Groundwater Elev. (ft)
7 7962.5 7960.0
8 7955.9 7953.1
9 7971.9 7970.3

VI. ALTERNATIVE PROJECT PLANS

Two different methods of lowering the water table and reducing subsurface flooding
problems were investigated. They are as follows:

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