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Storms create stresses on karst systems that can alter the pathways and traveltimes of water, solutes, and sediment. Flow contribution during storms is not only a matter of activation of new conduits, but is also a complex combination of water from conduits, enlarged fractures, and fractured matrix. In order to obtain evidence of pathway changes, we sampled three karst springs of varying size and maturity using data loggers for conductivity and water level, and storm water samplers for suspended sediment. The largest spring (Arch Spring) had the lowest conductivity of the three springs, indicating mainly conduit pathways at base flow. The high conductivity of base flow at the Nolte and Bushkill Springs pointed to contributions from slowermoving water in the fractured matrix. During storms, Arch Spring showed a consistent pattern of conductivity with a slight increase, then a large decrease, indicating an initial fracture flush of high-conductivity water, then passage of low-conductivity water from the precipitation. During storms, the conductivity of the middle-sized spring (Nolte Spring) either dropped immediately, or increased sharply then declined as storm water reached the spring. The smallest spring (Bushkill Spring) had a predictable conductivity pattern, with a sharp decrease and gradual recovery, suggesting shorter paths during storms than base flow. Sediment concentrations during storms were lowest at Nolte Spring and higher at Bushkill and Arch Springs, indicative of the fast flow through conduits or enlarged fractures suggested by the latter two springs during storms. The storm-water pathways vary from spring to spring and from storm to storm. These data show the importance of continuous monitoring to understand spring behavior.

Lititz Spring in southeastern Pennsylvania and a nearby domestic well were sampled for 9 months. Although both locations are connected to conduits (as evidenced by a tracer test), most of the year they were saturated with respect to calcite, which is more typical of matrix flow. Geochemical modeling (PHREEQC) was used to explain this apparent paradox and to infer changes in matrix and conduit contribution to flow. The saturation index varied from 0.5 to 0 most of the year, with a few samples in springtime dropping below saturation. The log PCO2 value varied from –2.5 to –1.7. Lower log PCO2 values (closer to the atmospheric value of –3.5) were observed when the solutions were at or above saturation with respect to calcite. In contrast, samples collected in the springtime had high PCO2, low saturation indices, and high water levels. Geochemical modeling showed that when outgassing occurs from a water with initially high PCO2, the saturation index of calcite increases. In the Lititz Spring area, the recharge water travels through the soil zone, where it picks up CO2 from soil gas, and excess CO2 subsequently is outgassed when this recharge water reaches the conduit. At times of high water level (pipe full), recharge with excess CO2 enters the system but the outgassing does not occur. Instead the recharge causes dilution, reducing the calcite saturation index. Understanding the temporal and spatial variation in matrix and conduit flow in karst aquifers benefited here by geochemical modeling and calculation of PCO2 values.