Major Ions dissolved in Water

        Pond water chemistry reflects the influence of watershed characteristics (e.g., bedrock, soils and land use). In this section we consider ions in largest supply in freshwaters (nutrients are in much smaller concentrations and are discussed separately in Section K). Ions are actually the charged constituents of salts that dissolve in water; for example, table salt is sodium chloride (NaCl), with one positively charged ion (Na+) and one negatively charged ion (Cl-) that dissociate in solution. The major positively charged ions of ponds in southeast Pennsylvania are calcium (Ca2+) and magnesium (Mg2+), with lesser amounts of sodium Na+ and potassium (K+). The major negatively charged ions are bicarbonate (HCO3-), carbonate (CO32--), sulfate (SO42-) and chloride (Cl-).
        Specific conductance is a measure of the total dissolved ion content of water, and is based on how well the water conducts an electrical current (the more the ions, the greater the specific conductance). Because Ca2+ and HCO3-/CO32-- are among the most important contributors to specific conductance, it should be no surprise that ponds found in the Chester Valley cutting across the middle of Chester County from northeast to southwest (Fig. 5) have high specific conductance values as a consequence of the limestone (CaCO3) underlying the valley floor. Specific conductance can be an excellent indicator of pond trophic state; ponds with higher specific conductance values are usually more productive because they contain not only higher concentrations of the major ions above, but also higher concentrations of nutrients.
        The concentrations of calcium and magnesium ions are measured together as “hardness”. Calcium and magnesium ions in ponds largely originate from limestones (see above) and dolomites ((CaMg(CO3)2) in the watershed. Water with hardness values of 0-60 mg/L as calcium carbonate is considered “soft”, values of 61-120 mg/L indicate “moderately hard” water, values of 121-180 mg/L indicate “hard” water, and water with hardness > 180 mg/L is considered “very hard”. Because calcium and magnesium contributed strongly to total ion content, a tight positive relationship between hardness and specific conductance is often observed in ponds of the region (Fig. 24).
Figure 24

Fig. 24. Relationship of hardness to specific conductance in 13 ponds (mean values based on visits during July, 2002 (values in March and May were similar). Dashed lines separate soft, moderately hard, and hard water. As indicated by the relatively little scatter around the regression line, hardness and specific conductance were closely related.

       The relative concentrations of positively and negatively charged ions help to determine the pH of pond water. Ponds with pH values < 7 are considered more acidic, while those with pH > 7 are more basic. “Alkalinity” measures the concentrations of negatively charged ions that collectively raise the pH above 7. The most common negatively charged ion is bicarbonate (HCO3-). Ponds with watersheds containing limestone may be expected to have higher alkalinity (and consequently higher pH) than other ponds of the county.
       Knowing the pH and alkalinity of a pond is important for two reasons. First, although most organisms characteristic of shallow ponds are able to tolerate a fairly wide range in pH, many algae have pH “optima” and most cannot tolerate severely acid conditions (e.g., pH < 5) resulting, for example, from acid rain or acid mine drainage. Ponds in Chester County typically have pH levels above 7, so concerns about acidification effects are likely minimal. Intense photosynthesis by algae and aquatic plants also elevates the pH, while respiration involved in the breakdown of organic materials causes pH declines. In effect, pH and alkalinity together can provide a strong indication of pond trophic state (highly productive ponds often have very high pH, especially during daylight hours).
       The 13 ponds sampled in 2002 ranged in pH from approximately 6 to nearly 10, and in alkalinity from 10 to 70 mg/L. Increased photosynthesis by algae and aquatic plants in July slightly elevated the values of both variables in most ponds (Fig. 25). In effect, for a given pond, identified by two-letter code, the general trend between March (open triangles) and July (closed circles) was up and to the right. An exception was pond BR, which already had large amounts of filamentous algae already growing at the bottom of the pond, and consequently high pH and alkalinity, in March.
Figure 25

Fig. 25. Relationship of pH to alkalinity in 13 ponds visited during March and July, 2002.