C ONCLUSIÓN

Long Bay, South Carolina experiences hypoxic and low dissolved oxygen events spanning hours to days that are unattributable to the typical formation mechanisms outlined by Diaz and Rosenberg (2008). Preliminary evidence showed increased concentrations of ²²²Rn in bottom waters during low DO events and an inverse

correlation between radon concentrations and DO levels, which suggests that SGD could play a critical role in hypoxia formation. McCoy et al. (2011) proposed a mechanism that wind patterns lead to limited cross-shelf mixing of warm, nearshore waters where inputs of SGD and terrestrially derived nutrients concentrate, increasing heterotrophic activity (Smith et al., 2010) and lowering DO. These authors were not able to definitively

determine whether the increased radon concentrations are indicative of increased SGD or decreased mixing. We hypothesized that radium isotopes would serve as additional tracers to delineate the effects of higher input rate versus lower output rate on observed radon concentrations and better understand the spatial and temporal variability of radium and DO dynamics, as well as development mechanisms of a hypoxic water mass.

Seasonal and time-series sampling of bottom waters during oxic and hypoxic conditions provided us with a comprehensive understanding of the inshore system. However, radium isotope activities during the 16-Aug-2012 hypoxic event could not be supported by the local beachface endmembers. We identified offshore groundwater sources (Moore

28

and Wilson, 2005; Moore, unpubl. data) with similar activity ratios of ²²⁸Ra/²²⁶Ra as our hypoxic samples. We calculated residence times using the offshore groundwater as endmembers and concluded that samples taken from 4-Aug through 14-Aug were of the same water mass, decaying in the nearshore and not mixing with lower activity water over a 10 day period. We also concluded that samples taken during the hypoxic event were of a different water mass than sampled in the previous weeks; minimum residence time calculations determined the hypoxic water mass was at least two weeks old. The significant differences in radium isotope activities during oxic and hypoxic conditions and the dissimilarity in calculated residence times suggest that a hypoxic water mass comprised largely of groundwater migrated inshore under the influence of certain physical conditions. After calculating the percentage of anoxic groundwater in each sampled water mass, we propose that hypoxic levels of dissolved oxygen in Long Bay may be independent of biological demand once offshore SGD occurs.

Future research should to be undertaken to identify if submarine groundwater discharge from the offshore wells is occurring under specific physical conditions, or is constantly occurring but normally mixing with lower activity waters. Once SGD drivers have been identified, we recommend further investigation into the physical conditions required to move the hypoxic water mass inshore, relatively undisturbed. Determining the extent of the hypoxic water mass in Long Bay will also add insight to movement capability. Not only do hypoxic conditions negatively affect aquatic life, but also can impact the local Grand Strand economy, thus adding increased importance to further investigation of hypoxia phenomena in Long Bay, South Carolina.

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36 TABLES

Table 1. Radon and radium isotope half-lives and properties.

Isotope Half Life Property Ra-224 3.6 days Solid Ra-223 11.5 days Solid Ra-228 5.8 years Solid Ra-226 1600 years Solid

Rn-222 3.8 days Gas

Table 2. Apache Pier established sampling station information. Average and standard deviation from the mean for each station during oxic conditions. Distance offshore from neap mid-tide water line. N = number of bottom water radium samples taken at each station from April 2012 – April 2013.

Sta. Distance

37

Table 3. Along-pier radium isotope average activities and one standard deviation for each weekly/monthly sampling event, n=5-6.

Date Ra-223 (dpm/L) xsRa-224 (dpm/L) Ra-226 (dpm/L)

38

Table 4. Radium isotope and physical parameter analysis r-values from time-series sampling of bottom water at Station 6 on 7-May, 19-Jul, 4-Aug, 14-Aug, and 16-Aug;

n=57, p<0.001, (n=51 for temperature).

xsRa-224 Ra-223 Ra-226

DO (mg/L) 0.816 0.887 0.894

DO Saturation (%) 0.878 0.787 0.894 Salinity (‰) 0.799 0.801 0.792 Temperature (⁰C) 0.871 0.857 0.871

*excludes 17-May-2012 time-series sampling event

Table 5. Percentage of groundwater composition (%), residence time (days) from ²²³Ra and ²²⁴Ra_xs, observed average dissolved oxygen concentration (mg/L), and calculated dissolved oxygen concentration (mg/L) for samples taken on 4-, 7-, 14-, and 16-Aug-2012. 16-Aug 47.92±10.21 17.17±3.73 13.97±1.25 3.27±0.66 3.27

39 FIGURES

Figure 1. Map of Long Bay, South Carolina. Star represents approximate location of Apache Pier (33⁰45’41” N, 78⁰46’47” W). Inset shows aerial photograph of Apache Pier.

40

Figure 2. Aerial photograph of Apache Pier in Long Bay, SC. Numbers on pier show approximate locations of sampling stations.

41

Figure 3. Bottom water physical parameter averages for each sampling station during oxic weekly/monthly sampling efforts. Error bars represent one standard deviation from the mean.

42

Figure 4. Radon and radium isotope averages (dpm/L) for each sampling station during oxic weekly/monthly sampling efforts. Error bars represent one standard deviation from the mean.

Figure 5. Seasonal radon and radium isotope activities (dpm/L) per station during oxic sampling conditions.

43

Seasonal radon and radium isotope activities (dpm/L) per station during oxic Seasonal radon and radium isotope activities (dpm/L) per station during oxic

Figure 6. Linear correlation plot of (A)

226Ra (dpm/L) with oxygen saturation (%) of bottom water at Apache Pier during five time-series sampling events at Station 6. R

44

Linear correlation plot of (A) ²²⁴Raxs (dpm/L), (B) ²²³Ra (dpm/L), and (C) Ra (dpm/L) with oxygen saturation (%) of bottom water at Apache Pier during five

series sampling events at Station 6. R² =0.619, 0.771, 0.799, respectively (n=57).

Ra (dpm/L), and (C) Ra (dpm/L) with oxygen saturation (%) of bottom water at Apache Pier during five

=0.619, 0.771, 0.799, respectively (n=57).

45

Figure 7. Average radium isotope average activities (dpm/L) during oxic and hypoxic conditions. Error bars represent one standard deviation from the mean.

46

Figure 8. Bottom water oxygen saturation (%) and radon activity (dpm/L).

Measurements at 30-minute intervals from 13-Aug-2012 through 21-Aug-2012.

R²=0.492, n=407, p<0.001. Data from the continuous monitoring station located on the end of Apache Pier.

47

Figure 9. Vertical water column profile of radium isotope activities (dpm/L), dissolved oxygen concentration (mg/L), and temperature (⁰C) taken at Station 6 on 17-Aug-2012.

48

Figure 10. Average radium isotope activities (dpm/L) from five beachface groundwater sampling events and bottom water hypoxic event. Error bars represent one standard deviation from the mean.

49

Figure 11. Ra-228 and ²²⁶Ra (dpm/L) activity ratios from ten offshore wells (Moore and Wilson, 2005; Moore, unpubl. data) and 16-Aug-2012 hypoxic samples.

50

Figure 12. Ra-228 and ²²⁶Ra activity ratios from Well A, Well LW, Well R, and 16-Aug-2012 hypoxic samples. R²=0.919, n=53, p<0.001.

51

Figure 13. Wind data from 1-Jul-2012 through 19-Aug-2012. Wind speed (m/s) and direction taken from the weather station on Apache Pier.

52

Figure 14. Residence time (days) of 4-Aug, 7-Aug, 14-Aug (shown in black), and hypoxic 16-Aug (shown in red) samples based on percentage of groundwater (%gw) and

223Ra activities (dpm/L). R²=0.999, n=3, p<0.001 for 4-, 7-, and 14-Aug-2012.

53

Figure 15. Residence time (days) of 4-Aug, 7-Aug, 14-Aug (shown in black), and hypoxic 16-Aug (shown in red) samples based on percentage of groundwater (%_gw) and

224Raxs activities (dpm/L). R²=0.998, n=3, p<0.001 for 4-, 7-, and 14-Aug-2012.

54

Figure 16. Observed and calculated dissolved oxygen concentrations (mg/L) for 4-, 7-, 14-, and 16-Aug-2012. Error bars represent the standard deviation from the mean.

Values above the marked bars represent average percent composition of groundwater used for calculated concentrations.

In document Sistema de gestion de Informacion de la Facultad 8. Modulo para la Gestion de la UJC y el Sindicato. (página 111-149)