This disclosure relates to systems, methods, and devices useful for monitoring and controlling water that has been used in, for example, oil and natural gas production, recovery, or hydraulic fracturing processes. An on-line unit for measuring, controlling, and optimizing the quality of produced water after being used in a hydrocarbon production or hydraulic fracturing process is also disclosed. Optimizing and controlling can include measuring one or more properties associated with the produced water to be sure that the one or more properties are within an acceptable range and, if the one or more properties are not within the acceptable range for each respective property being measured, causing a change in flow of one or more fresh water sources and/or one or more chemicals into the produced water.