Information and Data Collection Effort
In 2011, homeowners notified state officials of undesirable impacts related to high groundwater levels specifically in the Country Club Hills and Pawnee Ridge subdivisions in Sterling. The Governor's Water Policy Advisor, John Stulp, convened a number of meetings with stakeholders to understand the issues. In response to those meetings, the Colorado Water Conservation Board allocated funding for the Division of Water Resources (DWR) to undertake a multi-year project to gather the relevant data necessary to identify the factors contributing to the high groundwater levels in these areas and ultimately to commission an independent analysis and interpretation of the potential causal relationships leading to recommendations to mitigate those impacts. DWR staff have undertaken an effort to monitor groundwater levels; compile climate, diversion, and recharge data; and characterize the hydrogeology within the areas of interest to better understand the groundwater system. Preliminary information and data collected from this investigation is updated regularly to allow others access to the data and keep the stakeholders informed.
The links below direct the user to Adobe PDF documents in the areas of: 1) project background and geology; 2) updated climate, groundwater level, diversion, and recharge data; 3) studies; and 4) presentations.
December 2015 Climate Data
Precipitation is one of the main hydrologic inputs that influence groundwater recharge both as direct infiltration and water availability for diversion. To provide some perspective for the groundwater level data presented, we include graphs of precipitation from the Colorado Climate Data Center Sterling 1.8 NW station in the Pawnee Ridge area and the Northern Colorado Water Conservancy District’s Sterling #108 station located between US Highway 6 and the South Platte River just south of Sterling.
The scale of all precipitation graphs is the same to allow for comparison of precipitation events at the different locations. The first three months of 2015 were relatively dry. April and May made up for this trend with approximately 12 inches of precipitation, 7 inches of which occurred during May yielding the highest monthly value recorded during this study. June and July brought less precipitation than in years past, but the fall and winter months have yielded slightly higher than average precipitation.
Precipitation totals for 2015 in the Sterling area were 18.89 inches, just below the 2014 total of 19.68 inches and the 2013 total of 20.14 inches. Both 2015 and 2014 were wet years, well above the average annual precipitation of 15.13 inches. The impact of the larger precipitation events, in excess of one inch per day, is noticeable in some of the groundwater hydrographs, particularly the wells with shallow groundwater levels. The well itself may be responding to the wetting front migrating down to the water table, as the rise in groundwater levels quickly dissipates.
To provide a broader perspective, we have plotted the monthly total precipitation at the NCWCD #108 station. As mentioned during our presentations for this project, 2011 was the culmination of three wet years that would have impacted water management decisions in the area and thus groundwater levels. The higher than normal precipitation in 2013, 2014, and 2015 would also impact water management decisions in the area. In general, 2015 groundwater levels in many wells within the Study Area were the highest ever reported. Also included is a graph of annual precipitation to allow a year by year comparison.
The final graph is the Palmer Hydrological Drought index produced by the National Climatic Data Center for the Platte River basin in Colorado. The current graph compares the index for the entire year, January through December. Negative indexes in gold represent hydrologic drought years. The current graph includes 2014.
December 2015 Groundwater Level Data
To provide some perspective for the groundwater level data presented herein, a regional map is included that shows the locations of all of the wells (36) in which groundwater level measurements have been made. Water level monitoring in the southernmost cooperator well, Walker P#242904-A, has been discontinued at the owner’s request. Piezometers installed in the Country Club Hills and Pawnee Ridge subdivisions are equipped with electronic dataloggers that record water levels at hourly intervals. The Decision Support System well, DSS10STR, well #13 of the Town of Sterling well field, is also equipped with a datalogger. Other cooperator wells shown as yellow squares on the regional map are measured manually on a monthly basis by staff with the Division of Water Resources (DWR) or by staff of the Lower South Platte Water Conservancy District (LSPWCD). We greatly appreciate their assistance, and the cooperation from the respective well owners.
Project specific groundwater level information has been collected since May 2012. The latest piezometers were drilled and installed in March 2013. These are designated as “SGW-01 through SGW-04” on the following map. The new locations were sited to better understand water levels upgradient of the impacted areas. These new piezometers were instrumented with dataloggers in early June 2013. Also new to the monitoring network are: a piezometer installed by the Lower South Platte Water Conservancy District just downslope (east) of the North Sterling Canal (pz 1), and an old windmill well (pz 2) slightly further downslope (east) of the North Sterling Canal.
The second map in this section is a groundwater elevation contour map developed from the July 2013 water level measurements. Utilizing the well’s location of record, we extracted ground surface elevations from the USGS digital elevation model with a 10-meter resolution. Digital elevation models consist of a raster grid of regularly spaced elevation values derived primarily from the USGS topographic map series. Calculated elevations of the water table in July 2013 were used to create the groundwater elevation contour map. In general, the contours indicate that groundwater flow in this area of interest, west of the Town of Sterling, is to the east/southeast. Areas with greater well control such as the Country Club Hills and Pawnee Ridge subdivisions provide greater detail and indicate variability in the groundwater flow directions from northeast to southeast. All of the piezometers used for water level measurements were surveyed for elevation in early June 2015, however these more accurate elevations were not available at the time the groundwater elevation contour map was developed.
The groundwater level data collected by the dataloggers through January 6, 2016 are presented as hydrographs, where appropriate, in subsequent pages of this update and as manual measurements in tabular form. To facilitate analysis by stakeholders interested in this project, individual tables of the manual water level measurements are presented for Country Club Hills, Pawnee Ridge, the new SGW piezometers, and cooperator wells measured by DWR or LSPWCD staff. Water level data are graphed on an axis as depth below ground surface, which allows the viewer to gauge the depth and variability of the water table in relation to nearby structures. Original project piezometers were installed in three phases during April and May of 2012, and data loggers were deployed either on May 9 or May 31, 2012. Early data for well PRN-3 are limited due to a data logger malfunction requiring replacement of the device. Data for SGW-01 are missing since May 1, 2014 as the transducer/logger has fallen to the bottom of the well. Data are graphed using the same overall time scale and a standard depth range to facilitate comparison across the study area.
To provide for easier comparison of groundwater levels and trend analysis:
Hydrographs for piezometers installed within the Country Club Hill subdivision are plotted together and along the north-south and east-west transects.
Hydrographs for piezometers installed in the Pawnee Ridge subdivision and vicinity are plotted dependent upon whether the wells are located west or east of the Springdale ditch (i.e. north or south of CR30).
Hydrographs for the new SGW piezometers are plotted with other data in the subdivisions of interest or on a single graph with the PZ wells if further afield.
Hydrographs for data from cooperator wells are displayed individually (displayed from north [upland] to south [river valley]).
The graphed data are the depths to the water table below ground surface at the respective well location. This groundwater level may not represent the regional water table as the study piezometers are not completed through the entire aquifer interval. The presence of significant clay layers indicate that some of the measured water levels may represent locally perched water. The groundwater level data presented herein are best used to evaluate general trends in groundwater levels and responses to rain or other hydrologic inputs to the aquifer system.
One of the benefits of a groundwater monitoring network is the ability to record changes in the water table rather quickly. For example, data recorded in May 2014 document a localized change in water levels within a portion of the Country Club Hills subdivision related to a weed dam and resultant elevated stage within the Springdale Ditch.
Groundwater levels have been rising over much of the area through the summer, and have generally been declining throughout the fall. Maximum seasonal changes in non-pumping wells of approximately 4 feet (CCE-2) have been recorded. Review of the groundwater level data, collected to date, indicate:
- Prolonged rainfall events of several inches or more are required to detect a response in groundwater levels. It should be noted that the water table response to significant precipitation is a very short term rise with levels returning to the ambient equilibrium condition within days.
- In the Country Club Hills area, December 2015 groundwater levels vary from approximately 8 feet below ground surface to 27 feet. Water levels typically start to decline towards end of summer and rise again in late winter/early spring. This trend is seasonally consistent.
- The tabular data indicate that water levels are on the decline or are relatively stagnant as compared to November 2015.
- Water levels in all wells within Country Club Hills had been slowly declining from January to April, showed a sharp rise between April and May, a moderate decline through June followed by a rise in July, and since mid-July most are now on a steady decline. However in November, SGW-04 and CCE-2 showed a gradual rise while CCN-4 showed a dramatic increase. These rises may be related to the increased flow through the Springdale Ditch during the month of November.
- A comparison of year to year maximum (deepest) water levels indicates a rising trend in wells CCN-2, CCN-3, CCE-3, and CCE-4. This suggests that the aquifer in this area is not completely draining back to its previous year’s low.
- In the Pawnee Ridge area, December 2015 groundwater levels vary from less than 1 foot below ground surface to approximately 18 feet. The water level trend in wells north of CR30 (west of the Springdale ditch) remains remarkably similar even though the depth to water varies significantly.
- The tabular data indicate that water levels are were on the decline in every well throughout the summer. However, water levels during the fall show a relatively steep rise, particularly in PRE-1. The steady rise in water level measured at PRE-1 is likely due to the increase in flow to the Lebsock East and Lebsock West recharge ponds.
- Many of the hydrographs for monitoring wells in the Pawnee Ridge area show a steady trend of rising water levels over the study period. Currently, groundwater levels in all wells north of CR30 in the Pawnee Ridge area are at an all-time high. Residents in this area are currently having issues with water in basements.
- Water levels in SGW-02 and SGW-03 are also at historic high levels with a similar trend to the Pawnee Ridge wells. These rises correspond with diversions into the Monahan Recharge Pond. Additionally, the recent rise in the North Sterling Canal Piezometer #1 is due to the increased diversion volume through the North Sterling Canal in November and December 2015.
Water levels in some of the cooperator wells have a longer period of record and provide a historical perspective of water level trends in the area. We believe that the Lebsock #26087 is not completed in the alluvial aquifer, rather the underlying Pierre Shale bedrock. The groundwater level in these wells provides an understanding of the regional water table from upland to river valley and west to east.
December 2015 Recharge Data
The study area that encompasses both the Country Club Hills and Pawnee Ridge subdivisions contains a number of recharge ponds and diversion ditches. From west to east, the ditches include the North Sterling Canal, Pawnee, Springdale, and Sterling Lateral No. 1. These structures are shown and labeled on the following map of the Sterling Groundwater Pilot Project. Seepage from both the recharge ponds and ditches influence groundwater recharge and thus the groundwater levels. Unfortunately, historic return flows (seepage) from ditches are not quantified by the water commissioner unless that seepage is specifically credited for recharge. Consequently, this data only provides a partial picture of the recharge component.
To provide some perspective for how the recharge components of these structures may relate to the groundwater level data, we have included graphs of:
Total diversions at the headgates of respective ditches in both summary and individual charts. For comparative purposes, we include a bar chart comparing the annual ditch diversions (in acre-feet) for water years 2012-2016;
Amount of diversion credited as ditch recharge for the respective ditches in both summary and individual charts. For comparative purposes, we include a bar chart comparing the annual ditch recharge (in acre-feet) for water years 2012-2016;
Daily recharge to ponds for the respective ditches in both summary and individual charts. We also include a cumulative pond recharge chart for ponds associated with a specific ditch, and a bar chart comparing the annual pond recharge (in acre-feet) for water years 2012-2016; and
A chart plotting the flow rates through the Pioneer Park flume.
Some observations from these data include:
1. Since daily seepage values for ditches and canals are not available, the total amount of water being diverted at their headgate provides a qualitative assessment of both timing and amount of seepage. Generally, diversions to ditches started in mid-March for 2015. The annual diversion comparison clearly shows that the North Sterling Canal carries significantly more water than any of the other ditches. The North Sterling Canal did not divert in August or September.
2. The recharge credit value for ditch seepage applies to the entire length of the ditch, which may be several miles long. The actual ditch seepage or portion of total ditch seepage within the study area of interest is not known. The ditch recharge graphs, however, provide a timeframe for ditch seepage credit and groundwater recharge. Recharge credit is not applied when the ditch is flowing for irrigation. Since the beginning of 2015, all of the ditches have received recharge credit starting in April.
3. Daily recharge to ponds on both the Pawnee and Springdale ditches provides the schedule or timing for when these recharge ponds are filling. We do not have data on the volume of water within the structure, but the actual flow rate provides an indication of the amount of water diverted to recharge.
a. A cumulative pond recharge graph is provided that indicates the Monahan pond is the largest contributor to recharge along the Pawnee Ditch.
b. All three recharge ponds along the Springdale ditch started filling in April. The cumulative pond recharge graph shows that the Schuman pond is the larger contributor of the three ponds we track. The Schuman pond had been inactive since spring of 2013.