Monitoring hydraulic fracturing

This disclosure relates to systems, methods, and devices useful for monitoring and controlling water to be used, for example, in oil and natural gas production and hydraulic fracturing processes. An on-line unit for measuring, controlling, and optimizing the quality of water being used during the hydrocarbon production or hydraulic fracturing processes is also disclosed. Optimizing and controlling can include measuring one or more properties associated with the production 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 water sources and/or one or more chemicals.