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

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Regolith Groundwater Data Summary Groundwater level data collected from each of the three new regolith wells included in OWN depict the same general trend, namely a decrease in groundwater level from the initiation of da- ta collection in early June 2011 through the summer months until October to November 2011. Recharge of the regolith groundwater began in fall 2011 (October to November) and continued through April 2012. The hydrographs for the three regolith wells demonstrate that most of the recharge that took place occurred between mid-October 2011 and January 2012. Differences in the characteristics of recharge are apparent in the three hydrographs. Recharge of COL-1 appears to have been fairly steady through March, while the COL-4 hydrograph contains many rapid upward and downward changes in the groundwater level. The hydrograph for regolith well COL-3 contains three large peaks in the groundwater level with an approximately two month period (late December 2011 to March 2012) of very little change. Groundwater Data Discussion Usually, groundwater levels decrease during the months of the growing season, due to a reduc- tion in groundwater recharge. Groundwater recharge is reduced in part due to increased evap- oration as a result of higher ambient temperatures. Another important factor is the increased use of water by plants during the growing season (transpiration). These factors normally result in significantly lower groundwater levels during the summer months in North Carolina and else- where in the Northern Hemisphere. The non-growing season, November through March, is the period during which most significant groundwater recharge occurs. Recharge takes place when precipitation infiltrates into the ground and reaches the water table, rather than runoff, evaporate, or get used by plant life. This infiltration increases the amount of groundwater held in storage, raising groundwater levels in the regolith (upper) aquifer and then the underlying bedrock aquifer. Typically this recharge is then available for use during the subsequent grow- ing season. Periods of drought or dry weather can, of course, reduce recharge which could then impact the amount of groundwater available during the growing season. According to information posted on the Orange Water and Sewer Authority (OWASA) web site, as of April 25, 2012, 39.45 inches of precipitation had fallen in the previous twelve months at its Carrboro location, while the average annual precipitation in Carrboro is 45.72 inches. Thus 39.45 inches is 86 percent of the average annual amount for the preceding 12 months. The USGS raingage on Berry Andrews Road is in proximity of the three regolith wells that are in- cluded in OWN. This gage documented that 32.9 inches of precipitation fell at this location in the twelve months between April 26, 2011 and April 25, 2012, as shown in Figure 3. It is not known what the average annual precipitation is at this specific location, but it is likely to be in the vicinity of 45 inches. As a result, 32.9 inches of precipitation is probably significantly less than the amount of precipitation that usually falls in this area. Figure 4 is a summary of the percent of normal annual precipitation for the eastern half of North Carolina. This figure illustrates that Orange County received approximately 75 to 95 percent of the normal annual precipitation In total, this information indicates that Orange County experi- enced slightly decreased precipitation then is typical over the last twelve months. 7