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 CO ₂ subsequently is outgassed when this recharge water reaches the conduit. At times of high water level (pipe full), recharge with excess CO ₂ 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. ?? 2006 Geological Society of America.