taken in front of and behind the dune line, respectively, on the beach elevation; the "center 

 of dune" samples were taken west of the dune crest, about 5 to 10 feet above the beach 

 samples. Similar data from 1970 through 1972, excluding bay toe samples but including 

 "hurricane pass" samples from south Padre are given in Table 18. During heavy rainfall on 

 north Padre in the summer and fall 1973, water was often ponded behind the dune line. Soil 

 moisture generally remained higher behind the dune line than on the beach (Table 17). 



The relationship of subsurface soil moisture to monthly rainfall and surge inundations is 

 shown in Figure 25, for a backshore site (monthly plantings, about 4.5 feet MSL) over a 

 16-month period. Samples were taken at monthly intervals and also following surge 

 inundations. 



b. Soil Salinity. Soil salinity on the backshore varies, and is influenced by inundating 

 surges, salt-laden sand blown inland from the berm, salt spray, and rainfall. Generally, 

 subsurface salinity of the backshore, at elevations above 4.5 feet MSL, is relatively low and 

 usually remains so until a storm surge inundates the area. Hurricane washover passes on 

 south Padre are lower in elevation than a normal backshore, and are more subject to 

 flooding by minor surges which break through the berm. Drainage is also restricted, resulting 

 in frequent ponding of seawater followed by evaporation and concentration of the salts. 

 These processes leave the passes with a greater salt content than the higher backshore. 



Salt content is usually much lower on elevated surfaces of artificial or natural dunes, 

 especially beneath the surface where leaching and drainage is more complete, and regular 

 inundation (one source of salt) does not occur. Variations in elevated dune salt content are 

 due to salt spray and salt-laden drifting sand blown from the lower beach, especially after 

 high tides or surges. 



Surface salinities are usually higher than in the subsurface since soluble salts are brought 

 to the surface during periods of heavy evaporation. Surface salinity in hurricane passes 

 reach the highest values which explains why seedlings of even the very tolerant saltmeadow 

 cordgrass have not become established after over 2 years of protection by sand-fence 

 barriers from inundation. 



Measurements of salt content were made using a conductivity meter on samples of one 

 part ovendry sand thoroughly mixed and diluted with two parts of distilled water at 

 72° Fahrenheit. The term "soil salinity" is used in this study although conductance was the 

 unit of measurement. Conductance values, given in micromhos per centimeter, may be 

 converted to parts per thousand (ppt) of salt in the soil with the following formula 

 (Jackson, 1958): 



ppt salts in soil = (0.00064) (L) (2) 

 where 



L = specific conductivity in umho/cm. 



78 



