Agenda - 06-21-2011 - Information Item 1

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The groundwater hydrographs included herein are indicative of such a dry interval in Orange County. Dry hydrologic conditions have prevailed over the last twelve to fifteen months in the North Carolina Piedmont. According to information posted on the Orange Water and Sewer Authority (OWASA) web site, as of March 31, 2011, 35.97 inches of precipitation fell in the pre- vious twelve months at their Carrboro location, while the average annual precipitation in Carr- boro is 45.86 inches. According to the WRAL-TV web site, the fifty year annual average rainfall total for the Raleigh Durham International Airport is 42.53 inches, while the 2010 total was ap- proximately 37 inches. Figure 2 presents monthly percent of normal precipitation figures for 2011 from the NC State Climate Office. This information illustrates that Orange County has ex- perienced adecrease in the typical amount of precipitation in the last twelve to fifteen months. One consequence of the recent decrease in precipitation in Orange County is a reduction in the amount of water available to recharge groundwater. The limited amount of groundwater re- charge which has taken place since the 2010 growing season is demonstrated by the observa- tion well hydrographs presented above. These records clearly show that groundwater levels are significantly lower in early 2011 than they were in early 2010, indicating a noticeable de- crease in recharge. This may be of importance during the remainder of 2011 if dry conditions continue through the growing season. Once a greater amount of precipitation begins to occur, a time lag will pass before infiltration begins to reach the bedrock water table. As a result, it may take some time for an increase in precipitation, once it occurs, to raise groundwater levels in the Orange Well Net observation wells. It is important to note that further analysis of groundwater trends is constrained by the limited amount of data which has been collected to date. Groundwater level monitoring is a long-term process. United States Geological Survey (USGS) Circular 1217, entitled Ground- Water-Level Monitoring 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 obser- vation well and to track trends with time." Finally, the hydrographs collected #rom two of the observation wells appear noticeably different from the other four hydrographs. The Ray Road and Eno Confluence well hydrographs appear to be different from the other hydrographs, indicating 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 weN 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. Similarly, the well located at the Eno Confluence Property is located in proximity to both the West and the East Forks of the Eno River. The hydrograph for this well, included above, ap- pears to reflect a considerable amount of "flashiness" (rapid fluctuations, either up or down, in a short period of time) in the groundwater level. This is likely to be a result of fairly rapid in- creases and decreases in the flow of the Eno River as a result of precipitation in the upper Eno River watershed, indicating that the groundwater level in this area is fairly rapidly influenced by the water level in the Eno River. 7