The occurrence and geochemical characteristics of acidic groundwater in the Black Creek Formation in the vicinity of the town of Newton Grove, Sampson County, North Carolina / by Helin He.
| Author/creator | He, Helin author. |
| Other author | Woods, Terri L., degree supervisor. |
| Other author | East Carolina University. Department of Geology. |
| Format | Theses and dissertations |
| Production | 1994. |
| Description | ix, 111 leaves : illustrations (some color), maps ; 28 cm |
| Supplemental Content | Access via ScholarShip |
| Subjects |
| Summary | The Upper Cretaceous Black Creek Formation in the western portion of the North Carolina Coastal Plain was deposited in a deltaic to continental shelf sedimentary environment. It contains abundant iron sulfide in the form of pyrite/marcasite or the residue of decomposed pyrite. In the vicinity of Newton Grove and at some outcrops exposed near Benson and Clinton, the concentration of hydrogen ion in the sediments of the Black Creek Formation becomes so high that both the groundwater and surface water exposed to these sediments are acidified to below pH 3.5. The residents in Newton Grove are concerned about possible encroachment of acidic groundwater into their domestic wells. Abnormally rapid corrosion was also observed for pillars and concrete footings penetrating the Black Creek Formation. In this study, the vicinity of Newton Grove along Interstate 40 (1-40) was surveyed and four monitoring wells were drilled. In-situ measurements for chemical properties were performed at monitoring wells and streams. Aquifer tests were conducted to determine hydrologic parameters of the aquifer. Lithologic samples were collected from well sites and outcrops to conduct a laboratory simulation. The acidification of groundwater and surface water was probably caused by water exposure to sulfuric acid (H₂SO₄) derived from decomposition of pyrite and/or marcasite (FeS₂). The average pH readings were lower for the shallow wells (3.46 for well 701-15 and 4.04 for well 55-18) than deep wells (4.69 for well 701-26 and 5.02 for well 55-28). There was no apparent correlation between the pH of stream water at various sample localities and the distance of those localities from the highway. That is, highway construction does not appear to have affected the acidity of water occurring in the study area. Aquifer tests revealed that the horizontal hydraulic conductivities are 8-9 ft/day and 0.03-0.04 ft/day for the confined aquifer and confining bed, respectively, at well site 701. For the unconfined aquifer at well site 55, the horizontal hydraulic conductivity is about 4-6 ft/day and the vertical hydraulic conductivity is 0.7 ft/day or 1 1% of horizontal. Different patterns of pH variation were observed for surface water and groundwater. The higher pH values were usually observed in streams following a period of heavy rainfall, which could dilute the hydrogen ion concentration. In contrast, the lowest pH value of groundwater coincided with the highest water table elevation following a heavy rainfall, which indicates that vertical percolation might bring more hydrogen ions from the upper part of the aquifer into the well. These hydrogen ions are probably adsorbed on soil particles. Laboratory simulation demonstrated quantitively that limestone can neutralize the acidic groundwater from the sediments of Black Creek Formation. By exposing deionized water (pH above 5.5 and dissolved oxygen above 8.8 mg/L) to the lithologic samples from the Black Creek Formation, the dissolved oxygen in the final solution was reduced to below 4.4 mg/L accompanied with the deionized water acidified to pH below 3.5. It is obvious that the lower the pH of the final solution, the more dissolved oxygen has been consumed. Laboratory simulation combined with stratigraphic analysis revealed no conclusive correlation of specific lithologies with the [H⁺], [SO₄2-] and [Fe] of experimental solutions and groundwater. The techniques of microscopic examination (magnification less than 15X) and sediment sampling were probably inadequate to establish such a correlation. |
| General note | Submitted to the faculty of the Department of Geology. |
| General note | Advisor: Terri L. Woods |
| Dissertation note | M.S. East Carolina University 1994 |
| Bibliography note | Includes bibliographical references (leaves 73-75). |
| Genre/form | dissertations. |
| Genre/form | Academic theses. |
| Genre/form | Academic theses. |
| Genre/form | Thèses et écrits académiques. |
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