Agenda - 06-19-2012-13 (3) - Laserfiche WebLink

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One consequence of a decrease in precipitation is a reduction in the amount of water available to recharge groundwater. Evidence of reduced recharge is seen in the bedrock observation well hydrographs presented above. These records demonstrate that groundwater levels were lower in spring 2011 and spring 2012 than they were in spring 2010, indicating a decrease in recharge in 2011 and early 2012, when compared to the same time in 2010. It should be noted that further analysis of groundwater trends is constrained by the limited amount of data that has been collected to date. Groundwater level monitoring is a long-term process. United States Geological Survey (USGS) Circular 1217, entitled Ground-Water-Level Monitor- ing and the Importance of Long-Term Water-Level Data (2003) states; "Typically, collection of water-level data over one or more decades is required to compile a hydrologic record that en- compasses the potential range of water-level fluctuations in an observation well and to track trends with time." Once a hydrograph contains enough information, statistical evaluation of wa- ter levels can begin. Figure 5 is an example of the statistical information that is available on the DWR web site for the observation wells that are included in OWN. Graphs for all wells are available once a minimum of twelve months of data are posted to the web site, however, as more water level data is added to the record for each well, the statistical analysis is also updat- ed and as a result becomes more relevant. The reduced recharge that took place in early 2012 is probably not highly significant due to the amount of rain that fell in early May 2012 while this report was under preparation. However, it will take some time for this increased precipitation to raise groundwater levels in the Orange Well Net observation wells. Additional groundwater level data collection will document this. Several observations can be noted from the latest information presented herein. While re- charge was observed in all three regolith wells between November 2011 and April 2012, there seems to be variety in the patterns of recharge in these wells. Most of the larger (>1 inch) pre- cipitation events that took place between October 1, 2011 and the beginning of May 2012 took place before the end of December 2011, as shown in Figure 3. Variations in the responses to the larger precipitation events seems apparent amongst the three regolith observation wells. The groundwater level in regolith well COLA was mainly recharged in a steady gradual man- ner. The COL-3 hydrograph includes three large recharge events or steps. The COL-4 hydro- graph illustrates many more steps in the groundwater level than those observed in the COL-3 hydrograph. Of the three regolith wells, COL-3 is both the farthest from Collins Creek and at the greatest elevation above the creek. These factors may have bearing on some of the differ- ences in groundwater level records amongst the wells. Differences in lithology is also likely to be an important factor with respect to groundwater recharge of the regolith wells. It is also interesting to note that COLA does not seem to have experienced any appreciable recharge as a result of the nearly two inches of precipitation that fell from October 18th to 19th, 2011. It is possible that the quantity of precipitation at COLA was less than what fell at the oth- er regolith well locations, but since the three wells are within one-half mile of each other, this does not seem highly likely. Finally, the Ray Road and Eno Confluence bedrock well hydrographs appear noticeably differ- ent from the other bedrock well hydrographs. This indicates that factors that may not be im- portant at the other four wells may be influencing the groundwater level in these two wells. The Ray Road observation well is located in proximity of University Lake, an OWASA reservoir. It is possible that the groundwater level in this well is influenced by the water level in the nearby reservoir. Further analysis of the groundwater level in the Ray Road well would be needed to determine if the lake level is impacting this well. 8