Perhaps the most fundamental goal in education is achieving student retention of new information in order to accomplish learning outcomes. Scores of educational studies coupled with a plethora of books and papers over recent decades attempt to identify ideal educational practices which enable students to better retain material taught in the classroom. From these, numerous conclusions further support a range of influential factors that impact student retention, such as the method of instruction, classroom environmental conditions, relationship dynamics between students and teachers, and assessment or testing patterns. The focus of this study centers on this last factor, testing patterns, and how student retention can be improved in order to better accomplish learning outcomes. In our study, we examine and compare the overall value of periodic open and closednote formative assessments in an engineering course. We analyzed student performance for 202 students enrolled in an undergraduate environmental engineering course that covers several major topics to include environmental engineering fundamentals and chemistry, water and wastewater treatment methods and design, as well as air pollution modeling and control. This semester-long study included a comparison of student performance on major graded events, including midterm and course-end comprehensive examinations along with final term grades between class sections completing open-note, closed-note, and no additional formative assessments. Our study indicates that the type of testing, open versus closed-note, did not have a statistically different impact on overall course performance. However, the study did show a statistically significant increase of 2.97-4.87% in course performance and final averages between sections completing either type of periodic testing, versus sections completing no additional testing. This finding suggests that formative assessments not only serve to achieve better retention in an environmental engineering course, but further support current academic literature asserting that testing in the classroom generally results in improved student performance.