The application relates to a sustainable and green remedial approach for in situ remediation. The system and method use directionally drilled horizontal wells filled with granular reactive media generally installed in the direction of groundwater flow for groundwater remediation. “Flow-focusing” behavior is leveraged to capture and passively treat proportionally large volumes of groundwater in situ. The system and method perform well in low hydraulic conductivity environments. Reactive media are selected according to the contaminants to be treated and site characteristics The application further relates to a source area bypass system comprising one or more horizontal wells, constructed in a manner to allow unimpacted or cleaner groundwater to bypass a nonaqueous phase liquid zone or high-concentration source area of contamination and discharge downgradient. Reactive media may or may not be used in a source area bypass system. In some configurations, groundwater pumps may be installed to enhance performance.