In a large bowl, whisk together the water, ketchup, brown sugar, orange juice concentrate, soy sauce, mustard, salt, garlic powder, ginger, and pepper until smooth.
Pour half the marinade into another large bowl or dish. Add the chicken pieces and turn to coat. Cover and refrigerate for at least 8 hours or overnight for maximum flavor.
Store the remaining marinade separately in the refrigerator.
Discard the used marinade from the chicken. Place the chicken pieces into a 4 or 5-quart slow cooker, then pour the reserved marinade over the top.
Cook on low for 7-8 hours, until the chicken is fully cooked (juices should run clear when pierced).
Remove the chicken from the slow cooker and transfer to a serving platter, keeping it warm.
Skim the fat from the cooking juices in the slow cooker and transfer the liquid to a small saucepan. Bring the sauce to a boil on medium heat.
In a small bowl, mix the cornstarch with cold water until smooth. Gradually whisk this mixture into the boiling sauce.
Continue cooking the sauce, stirring constantly, for about 2 minutes or until thickened.
Add the drained mandarin oranges, green pepper, and olives (if using) to the sauce and stir until combined.
Spoon the sauce over the chicken and serve with hot cooked rice for a complete meal.